"When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action," Haley said. "For the sake of the victims, I hope the rest of the council is finally willing to do the same."
Haley, who is also the UN Security Council president, was speaking as the council considers a resolution condemning the Assad regime for the attack, which killed dozens, including many children. Russia, which backs the regime of Syrian President Bashar al-Assad and has troops in Syria, is likely to veto the resolution.
"How many more children have to die before Russia cares?" Haley said, leaving her presidency chair as she displayed photos of the victims.
For now, Haley's comments are the most direct threat of unilateral action by the US about acting on the Syria crisis -- but they mark a departure from the Trump administration's more hands-off stance on Syria and may be more rhetoric than reality.
Trump, speaking at the White House during a visit by Jordan's King Abdullah, said Wednesday that "we see what happened just recently yesterday in Syria horrible. Horrible, horrible thing. Unspeakable."
First real test for Trump in Syria
Assad will be watching to see if that's the extent of Trump's response, experts say. The gas attack marks the Syrian strongman's first real test of Trump, who repeatedly criticized former President Barack Obama for getting involved in Syria. In the immediate aftermath of the gas attack, the White House released a statement blaming Obama for the tragedy.
Secretary of State Rex Tillerson told a March gathering of the international coalition to defeat ISIS that "there are many pressing challenges in the Middle East, but defeating ISIS is the United States number one goal in the region." That was interpreted by many to mean that the Obama administration goal of removing Assad from power and pushing for a political solution to the Syrian war had been set aside.
Indeed, during a late March visit to Turkey, Tillerson said that Assad's fate "will be decided by the Syrian people," language that Syria, Iran and Russia have used to convey their intention that the Syrian leader isn't going anywhere. The next day, White House spokesman Sean Spicer said that "with respect to Assad, there is a political reality that we have to accept in terms of where we are right now."
Critics such as Arizona Republican Sen. John McCain told CNN on Wednesday that Tillerson's comment was "one of the more incredible statements I've ever heard" and that he was sure that the Assad regime was "encouraged to know that the United States is withdrawing" from the conflict.
Sen. Marco Rubio, R-Florida, went a step further when he said Wednesday that he doesn't think it's a coincidence that a suspected chemical weapons attack in Syria occurred shortly after Tillerson suggested Assad could remain in power.
"In this case now, we have very limited options and look, it's concerning that the secretary of state, 72 hours ago or a week ago, last Friday, said that the future's up to the people in Syria on what happens with Assad. In essence, almost nodding to the idea that Assad was going to get to stay in some capacity," Rubio said on the show "AM Tampa Bay."
"I don't think it's a coincidence that a few days later we see this," Rubio concluded.
Tillerson has said that the administration backs the idea of creating "interim zones of stability" within Syria where refugees could go, but he didn't offer any detail on a plan that would require defending those zones and therefore heighten the risk of military clashes with Russia.
The administration could explore other options, said Daniel Serwer, director of the Conflict Management Program at Johns Hopkins University.
The US could go forward with the plan to declare and defend safe areas, Serwer said. It could identify and destroy the aircraft or artillery involved in launching the chemical weapons; it could launch an air attack on the Syrian and allied ground forces advancing on opposition-controlled areas. Trump could also push Moscow to stop Syria's use of chemical weapons, he added.
"So far, Donald Trump has said this cannot be ignored by the civilized world, but has done nothing," Serwer said, adding that "Trump's failure to act is a green light for Assad to do as he likes."
Security Council eyes resolution
Representatives from the US, United Kingdom and France have circulated a draft resolution demanding that the Syrian government provide flight logs from Tuesday, the day of the attack, which killed dozens, including many children. The draft also calls for meetings with Syrian generals or other officers.
The resolution explicitly condemns the use of chemical weapons within Syria and states that their use represents a threat to international peace and security.
An emergency Security Council meeting was called at UN headquarters in response to the suspected targeted attack.
The Syrian military under Assad has denied responsibility for the attack, claiming it would never use chemical weapons.
Multiple countries denounced the attack on Wednesday before and during the Security Council hearing.
Bahram Qasemi, a spokesperson for Iran's Foreign Ministry, said the country "strongly condemns" the use of chemical weapons in Syria "regardless of the perpetrators and the victims."
France's UN ambassador, François Delattre, said of the attack: "That reminds us of the day of the regime's attack on Damascus
. This chemical horror opens a new spiral into the abyss of the human tragedy."
But Russian Foreign Ministry spokeswoman Maria Zakharova blasted the resolution, telling reporters Wednesday it was "unacceptable" and that it was drafted too quickly.
The UK and Chinese ambassadors also exchanged sharp words over responsibility for UN inaction.
UK Ambassador Matthew Rycroft condemned prior vetoes by China and Russia, which blocked international action on Syria. But China's Ambassador Liu Jieyi, in a rare diplomatic outburst, said the UK delegate should stop "abusing" the Security Council.
CORRECTION: An earlier version of this story misquoted Haley.