Barack Obama's ambassador to the United Nations said on Sunday that reports of a poison gas attack in Syria, in a rural village north of Damascus, were so far "unsubstantiated", and added that the US was trying to establish what happened before it considered a response.

"We are trying to run this down," said Samantha Power, the US ambassador to the UN, during an appearance Sunday on ABC's This Week.

"So far it's unsubstantiated, but 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," she said.

Online videos of the attack were reminiscent, albeit on a much smaller scale, of a 21 August chemical attack near the Syrian capital, Damascus, that killed hundreds of people.

The US and its allies blamed Bashar al-Assad's government for that attack, which crossed a "red line" that Obama had said would bring harsh consequences. The attack nearly sparked western airstrikes before a negotiated diplomatic settlement saw Assad's government agree to give up its chemical weapons. Damascus denied the charges and blamed rebels of staging the incident.

About half the weapons have been removed from Syria so far. The Syrian government has missed several deadlines, blaming the delays on security concerns.

Both sides in Syria's civil war blamed each other for Friday's alleged attack, that reportedly injured scores of people.

The details of what happened in Kfar Zeita, an opposition-held village in Hama province some 125 miles north of Damascus, remain murky. Online videos posted by rebel activists showed pale-faced men, women and children gasping for breath at what appeared to be a field hospital. They suggested an affliction by some kind of poison and yet another clouded incident where both sides blame each other in a conflict that activists say has killed more than 150,000 people with no end in sight.

Opposition groups, including the main western-backed Syrian National Coalition, said the poison gas attack hurt dozens of people, thought it did not identify the gas used. State-run Syrian television blamed members of the al-Qaida-linked Nusra Front rebel group for the attack, saying they used chlorine gas to kill two people and injured more than 100. It did not say how it confirmed chlorine was used.

The opposition also has claimed other, limited use of chemical weapons or poisonous gas attacks near Damascus in recent days.

Power's comments came as heavy fighting raged Sunday across many parts of the country. In the war-shattered northern city of Aleppo, activists said at least 29 people were killed over the weekend.

The Britain-based Observatory for Human Rights (OHR) said that at least 16 rebels were among those who died in the overnight combat. At least 13 civilians also were killed when government aircraft dropped barrel bombs on the city's rebel-held districts.

Another activist group, the Syria-based Local Coordination Committees, said Assad's warplanes launched fresh airstrikes there on Sunday.

Aleppo, Syria's largest urban center and its one-time commercial hub, has been a key front in the civil war. The fighting has been in a stalemate for months.

Both activist groups also reported airstrikes on rebel positions in a village in the oil-rich Deir el-Zour province near the Iraqi border. The OHR said the strikes killed at least four people and wounded scores.