Russia's increasing military action in Syria is forcing moderate elements within the Syrian opposition into the hands of extremists, reported the BBC citing a US state department spokesman.

Mark Toner said renewed Russian and Syrian air strikes were making a difficult situation "more confused".

He pointed out that a recent attack on an aid convoy, which the US blames on Russia, and renewed air strikes on Aleppo were driving moderate rebel groups closer to the extremists.

"What has happened now, with the hitting of the humanitarian convoy and with the subsequent siege on Aleppo, you've got a scenario now, a dynamic where, as these moderate opposition forces are under increasing pressure from the regime, that they are driven into the arms [of al-Nusra], and they have to fight side by side," he stated.

"It escalates, and makes more confusing, what is already a difficult situation," he added.

Meanwhile Russia has accused the US of trying to spare a jihadist group in its bid to oust Syria's President Bashar Al Assad.

Russian Foreign Minister Sergei Lavrov said the US had broken its promise to separate the powerful Jabhat Fateh al-Sham (formerly known as al-Nusra Front) and other extremist groups from more moderate rebels.

Toner dismissed the Russian allegations as "absurd".

He told reporters that the US had not targeted Al Nusra for months because they had become "intermingled" with other groups and civilians.

On Friday, Syrian government forces and rebels fought in the centre of Aleppo and north of the city, a week into the Russian-backed offensive by the Syrian army to take the city and surrounding area.

Monitoring group the Syrian Observatory for Human Rights and a Syrian military official said government forces had captured territory north of Aleppo and buildings in the city centre, but rebel sources denied this.

There are many armed rebel groups fighting against the government of Bashar Al-Assad, said the report.

The recent US-Russian cessation of hostilities deal was meant to lead to joint Russian-US air strikes on the Islamic State group and Jabhat Fateh al-Sham, which is linked to al-Qaeda.

But many of the more moderate rebel groups that the US backs have formed a strategic alliance with Jabhat Fateh al-Sham and now fight alongside it.

The UN says 400 civilians, including many children, have been killed in the besieged city of Aleppo during the past week as a result of bombardment by Russian and Syrian government forces.