That action, if it took place, not only violates U.S. export laws but also raises further questions related to why lawmakers and military officials condone the sale and use of this controversial weapon.
The weapon -- which not only delivers an initial explosion on impact but also contains multiple smaller bombs that spread over a wide area -- is largely condemned by the international community due to the risk of civilian casualties when used in populated areas.
In its report, Human Rights Watch concluded that the Saudi-led coalition is responsible for all or nearly all the cluster munitions attacks in Yemen because it is the only entity operating aircraft capable of delivering the weapon.
Cluster bombs can be legally exported under U.S. law and have been sold to Saudi Arabia and the United Arab Emirates in recent years.
But recipients must also agree not to use them in civilian areas, according to the law. And in 2008, the United States put in place stringent requirements on the reliability rate of the weapons.
U.S. reviewing report
U.S. officials have acknowledged reports of civilian casualties
in Yemen but stopped short of specifically recognizing the use of cluster munitions or accepting any direct responsibility for collateral damage resulting from coalition strikes.
"We are aware of the Human Rights Watch report and are reviewing it," said Christopher Sherwood, a spokesman for the Department of Defense.
The Saudi Arabian government did not respond to a request for comment. But in a January interview with CNN's Nic Robertson, Brig. Gen. Ahmed Asiri, a Saudi military spokesman, said Saudi Arabia has used cluster munitions in Yemen against concentrated rebel camps and armored vehicles but not in cities populated with civilians.
In the January interview, Asiri specifically denied reports that Saudi Arabia used cluster munitions on a civilian neighborhood in the city of Sanaa.
"We don't use cluster bombs in Sanaa, this is clear and definite," he said.
But on Thursday, the European Parliament called on the European Union
to impose an arms embargo on Saudi Arabia in response to allegations of human rights violations in Yemen.
"Our duty is to the civilians in Yemen, and given widespread and very valid concerns over the conduct of the war by Saudi forces, our call for an E.U.-wide arms embargo is proportionate and necessary," said Alyn Smith, one of the lawmakers who spearheaded the vote.
"The Saudis say they are investigating what they call incidents, and I welcome that scrutiny. But that is not enough -- humanitarian NGOs have asked for an independent inquiry that meets international law standards," he said.
Prince Mohammed bin Nawaf bin Abdulaziz, Saudi Arabia's ambassador to Britain, responded to the calls for an arms embargo Monday in an op-ed in The Telegraph
, insisting the Saudi campaign in Yemen is in "full compliance with international humanitarian law."
"Precision weapons are being used over cluster munitions, and targets are thoroughly examined to ensure the avoidance of civilian casualties," he wrote.
The Human Rights Watch report, published earlier in February, includes interviews with witnesses and victims. Photographs and video evidence, the human rights groups said, confirm that the Saudi-led coalition, which includes the United Arab Emirates, Kuwait, Bahrain, Qatar and other countries, is using cluster munitions in civilian areas as part of its military operations against Houthi rebels.
"Saudi Arabia and its coalition partners, as well as their U.S. supplier, are blatantly disregarding the global standard that says cluster munitions should never be used under any circumstances," said Steve Goose, arms director at Human Rights Watch and chairman of the international Cluster Munition Coalition. "The Saudi-led coalition should investigate evidence that civilians are being harmed in these attacks and immediately stop using them."
One strike documented in the report occurred on December 12, when witnesses said coalition aircraft dropped cluster munitions on the military port of al-Hayma.
While the Yemeni coast guard and Houthi rebels both occupy parts of the port, it is close to a local fishing village, according to the report.
That morning, a witness said he saw aircraft drop bombs that exploded and then released a series of smaller bombs.
Because of the way the wind was blowing, the smaller bombs -- delivered by what were described as "parachutes" -- started to fall toward the witness's village.
One of those parachutes hit the house of the witness's brother, sending metal fragments flying into a room and injuring three people inside, including a 4-year-old girl.
Doctors had to amputate the leg of one of the men inside the home because of injuries he suffered in the attack, the witness told Human Rights Watch.
The report documents more than a dozen other witness accounts of similar explosions during airstrikes in the country.
Military planners monitor Yemen
The United States has a small group of military planners who monitor the situation in Yemen to act in an advisory role and help share intelligence.
But officials said the United States does not have a direct hand in coordinating or carrying out military strikes in the conflict.
"We are confident that the intelligence and advice we pass on to Saudi Arabia and other coalition members is sound, giving them the best options for military success consistent with international norms and mitigating the potential for civilian casualties," said U.S. Navy Cmdr. Kyle Raines, a spokesman for U.S. Central Command.
"The final decisions on the conduct of operations in the campaign are made by the members of the Saudi-led coalition, not the United States," he added.
Cluster munitions are delivered from the ground by artillery and rockets or dropped from aircraft. They contain multiple smaller submunitions that spread over a wide area.
The threat these munitions pose to civilian populations at the time of an attack, as well as the tendency of the submunitions not to explode and remain active until they are cleared or destroyed, prompted 118 countries to ban their use and join the Convention on Cluster Munitions in 2008.
While the United States is not a member of the Convention on Cluster Munitions, it does have an export law that prohibits recipients of cluster munitions from using them in populated areas, stating they "will only be used against clearly defined military targets and will not be used where civilians are known to be present or in areas normally inhabited by civilians."
U.S. law also mandates that cluster munitions only be sold if they demonstrate a failure rate -- i.e., leaving unexploded ordnance behind -- of less than 1%. Human Rights Watch's report said that reliability standard is not being met by those used in Yemen.
U.S. military officials "have and continue to recommend to the Saudi military leadership to investigate all incidents of civilian casualties allegedly caused by airstrikes and has asked that the coalition reveal the results of these investigations publicly," according to U.S. spokesman Raines.
But human rights groups have called for President Barack Obama to follow the EU Parliament's lead and take a deeper look at the United States' role in the growing civilian death toll in Yemen.
"The Obama administration is basically tolerating civilian casualties in Yemen and giving lip service that they are concerned," said William Hartung, an adviser to the Security Assistance Monitor.
The Department of Defense said that it remains "deeply concerned by reports of harm to civilians" and that the United States has "encouraged the Saudi-led coalition to investigate reports of civilian harm," according to Sherwood.
But Hartung charged that the United States looks the other way on reports of civilian casualties and continues to sell cluster bombs to the Saudis because of a number of geopolitical and economic factors.
First, by backing the Saudi campaign in Yemen, the United States is attempting to reassure Riyadh that the nuclear deal with Iran doesn't indicate a tilt toward regional support for Iran, which backs the Houthis.
History of arms sales to Saudi Arabia
He also said the United States' long-standing role as an arms supplier to Saudi Arabia has been very lucrative for American companies.
The Defense Department maintained the sale and use of this particular type of weapon isn't a problem if they are used under the appropriate guidelines.
"We have consistently reinforced to coalition members the imperative of target analysis and precise application of weapons in order to identify and avoid structures and areas that, if struck, could result in civilian casualties," Sherwood said.
Despite reassurances from U.S. officials, human rights groups hope they can force the United States to reassess its policy of selling cluster munitions, specifically to Saudi Arabia, by continuing to apply public pressure.
Reports of civilian casualties in Yemen resulting from the use of cluster munitions have generated some conversation among lawmakers on Capitol Hill.
In July, Rep. Jim McGovern, a Massachusetts Democrat, raised concern about the use of these weapons in Yemen.
"If we have evidence that countries are not complying with U.S. law, that ought to be enough to say we sell these weapons to them no more. Period. End of story," McGovern said, adding that the United States should join the Convention on Cluster Munitions.
McGovern is co-sponsor of the Cluster Munitions Civilian Protection Act, a bill he has been trying to make law since 2007.
The Senate, at this point, is unlikely to pass McGovern's bill or join the anti-cluster bomb treaty, but Hartung said the Obama administration could still abide by its rules without formally becoming part of the group.
The more these casualties are documented, the more leverage human rights groups have over the administration, Hartung said: "While it is unlikely they will stop the large sales, President Obama could re-evaluate what is being used in Yemen."