Syrian warplanes have bombed northern Aleppo for a fifth consecutive day, with opposition activists alleging that government forces dropped barrel bombs - barrels filled with explosives dropped from helicopters - on the city and several nearby villages.

The attacks on Thursday killed at least 11 people in two villages, among them four women and two children, according to the Syrian Observatory for Human Rights.

"I think it is trying to make the population turn against the rebels, so that the people themselves expel the fighters," Rami Abdel Rahman, director of the Britain-based Observatory, said.

One Syrian security source denied that barrel bombs were used, but another said the Syrian military prefers such weapons over missiles because they are cheaper, according to the AFP news agency.

In a statement on Wednesday evening, Doctors Without Borders said that government air strikes had killed at least 189 people and injured 879 others in Aleppo since December 15.

The US this week drafted a UN Security Council statement condemning the government's increasing military offensive on Aleppo, but the proposed text was blocked by Russia, diplomats said.

Russia, the Syrian government's main ally, wanted all references to the regime stripped from the statement so the US decided to drop it, according to diplomats at the UN.

The statement would also have condemned violence by all parties in Syria and expressed deep concern at the escalating level of violence in the Syrian conflict, including the use of Scud missiles and "barrel bombs" in Aleppo.

The city has been locked in a stalemate for more than a year, after a massive advance by opposition fighters in July 2012. Since then, parts of the city have remained under government control, while others have been in opposition hands.

The villages targeted have been under the control of opposition fighters and have suffered frequent bombing.

The air campaign comes as world powers prepare for an international peace conference planned for late January in Switzerland.

The opposition National Coalition said the "systematic raids on Aleppo demonstrate the regime's rejection of a political solution".

More than 120,000 people have been killed, and millions forced from their homes, in Syria's 33-month war.