(CNN) -- The Obama administration and its allies believe the Syrian government may have used chlorine gas in a deadly attack this month on its own people, several U.S. officials and other diplomats told CNN.
The alleged assault that killed at least two and affected dozens of others occurred in the village of Kafr Zeita, a rebel-held area.
While there is no firm proof as the matter is being looked into, several U.S. officials and Western diplomats say the United States believes the regime of Bashar al-Assad is responsible because it has such chemicals and the means to deliver them.
"Our assessment is it is, at a minimum, concentrated chlorine dropped from helicopters," a U.S. official said. "That could only be the regime."
The official did not speak for full attribution.
If true, such an attack would spotlight a deal brokered by Russia last fall and approved by the U.N. Security Council that requires Syria to surrender its chemical weapons to the international community.
The agreement quieted threats of military action by the United States over allegations Syria launched a chemical attack last August outside Damascus that killed more than 1,400 people. Al-Assad and other officials vehemently denied their forces were responsible.
The Syrian opposition, which does not have helicopters to carry and deliver such weapons, and the regime have been trading accusations about the April 11 incident for more than a week.
Controversy followed video clips posted on anti-government websites showing a number of civilians, including children, appearing to have difficulty breathing and using oxygen masks.
The chemical symbol for chlorine (Cl2) is visible on the side of a canister that opposition activists say was used in the attack.
"There are indications that these substances maybe have been used by the regime, but it has to be verified," one Western diplomat said.
State Department spokeswoman Jen Psaki conveyed Western suspicions on Monday.
She said the United States "had indications of the use of a toxic industrial chemical, probably chlorine, in Syria this month, in the opposition-dominated village of Kfar Zeita."
"We're examining allegations that the government was responsible," Psaki said.
Chlorine is not listed as a chemical Syria is expected to give up under the Security Council resolution. But its use as a weapon of war is prohibited under the 1925 Chemical Weapons Convention, of which Syria is a signatory.
"You don't have to use sarin and mustard to terrorize a population," another official said of deadly chemical gases. "And you don't have to kill a lot of people to terrorize the population."
U.S. officials and Western and Israeli diplomats point to several incidents over the last month suggesting the use of toxic gases, seen as particularly effective in sowing panic and fear in rebel-held areas.
Officials say there now is a debate within the administration about how to respond.
The matter is likely to be referred for investigation to the Organization for the Prohibition of Chemical Weapons, the global body charged with eliminating Syria's stockpiles.
The latest revelation poses a new dilemma for the administration on Syria, where civil war continues to rage. While chlorine is not as poisonous as chemicals like mustard gas or sarin, it would violate of Syria's international obligations.
While some of the administration feel a strong response is needed, officials said others are concerned a robust response would complicate ongoing cooperation with Syria on its more dangerous stockpiles.
"There is not a consensus," another U.S. official said. "Some would like to be stronger and some are more cautious. It can't go answered but if we equate it to a chemical attack then the question becomes what are we going to do about it? And I don't think we have figured that out yet."
Sen. John McCain, who has been sharply critical of the administration's Syria policy, said in a statement the latest allegations are troubling.
"It is essential that the Obama administration work with the international community to fully and immediately investigate these reports. If substantiated, it is clear that such attacks violate the spirit of the U.S. agreement with Russia for the destruction of Syria's chemical weapons and the Assad regime must finally be held accountable for it actions," the Arizona Republican said.