The Syrian army has entered the province of Raqqa during a Russian-backed offensive against Islamic State, according to reports.

Heavy Russian airstrikes also bombarded IS territory east of nearby Hama province, according to the Syrian Observatory for Human Rights (SOHR), a UK-based group that monitors the war.

On Friday, US-backed Kurdish Syria Democratic Forces (SDF) took control of four villages en-route to Manbij, an IS-held city in the of province of Aleppo, according to local monitors.

The SOHR said it also received information that 10 civilians, including five family members, were killed in US-led coalition airstrikes in Ojana village near Manbij.

Some 20 IS fighters and four from the SDF have been killed in three days of fighting, the Observatory added.

The ongoing fighting comes as the United Nations Security Council announced it will formally ask Syria to allow humanitarian air drops to besieged areas.

UN humanitarian chief Stephen O'Brien warned that access to millions of Syrians in need of help is deteriorating amid the fighting.

Taking Raqqa from IS is a major strategic goal for those seeking to topple the Islamist terror group and reduce its stranglehold on Syria.

The five-year Syrian civil war has killed more than 250,000 Syrians. Millions more have been forced from their homes.

A fragile, US-Russian brokered truce was implemented in February but ferocious fighting has continued to rage in many parts of the country killing many hundreds of soldiers and civilians.