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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.