BEIRUT - The United States said Sunday that it was looking into claims of a new chemical-weapons attack in Syria, after rebels and the government traded accusations over the use of chlorine gas in a central village.

The alleged attack on Kafr Zeita, a rebel-held village 125 miles north of the capital, Damascus, has not been independently verified but has been acknowledged by both sides.

"We are trying to run this down," Samantha Power, the U.S. ambassador to the United Nations, told ABC's This Week. "We've shown, I think, in the past that we will do everything in our power to establish what has happened and then consider possible steps in response."

But she said the claims so far are "unsubstantiated."

Both government and pro-opposition sources have said that chlorine gas was used in the attack Friday on Kafr Zeita, which has been the site of heavy clashes in recent weeks.

If the Syrian government is implicated, it could ramp up pressure on the United States to take more robust actions against Damascus. The Obama administration backtracked on plans for air strikes after an Aug. 21 chemical attack in the Damascus suburbs in favor of a program under which Syria would give up its chemical weapons. The program is running behind schedule.

Syrian rebels and opposition activists say the toxic gas was dropped by helicopter, and they have circulated a video that they claim shows the moment of the attack. Another video showed men being treated with oxygen and children coughing and crying.

"Tens of children were poisoned and choked," said a man dressed in green medical scrubs.

The opposition Violations Documentation Center (VDC), which tracks human-rights violations, said the chemical attack occurred about 6 p.m. Friday. It said a strong smell of chlorine hung in the air within a 1,600-foot radius of where the helicopter struck, citing a doctor who treated the injured.

The chemical symbol for chlorine, Cl2, is visible on the side of a canister pictured in photographs distributed by opposition activists. They say the image shows the remains of the delivery device.

Chlorine gas, which was widely used during World War I, can be deadly. But the VDC said that only two people died in the Kafr Zeita attack - a child and an elderly man - and that the deaths were from the impact of the explosions rather than the gas. About 100 people exhibited symptoms that included breathing difficulties and vomiting, the group said.

It is a latest in the string of relatively small-scale chemical attacks reported by the opposition, which it says have largely focused on the Damascus suburbs.