With a UN envoy due in Damascus in a bid to advance struggling diplomatic efforts, the "cessation of hostilities agreement" brokered by Russia and the United States came under new strain as government and rebel forces fought near Aleppo.

The ceasefire came into effect in February with the aim of paving the way for a resumption of talks to end the five-year-long war. But it has been widely violated, with each side blaming the other for breaches. The fighting south of Aleppo marks the most significant challenge yet to the deal.

Diplomacy has meanwhile made little progress with no compromise over the future of President Bashar al-Assad, his position strengthened by Iranian and Russian military support.

A top Iranian official, in comments to Iran TV, rejected what he described as a US request for Tehran's help to make Assad leave power, saying he should serve out his term and be allowed to run in a presidential election "as any Syrian".

Syrian Prime Minister Wael al-Halaki told a delegation of visiting Russian lawmakers of preparations to "liberate" Aleppo, Syria's biggest city and commercial hub before the conflict that erupted in 2011. Aleppo is divided into areas controlled separately by the government and opposition.

Dmitry Sablin, a member of Russia's upper house of parliament and a member of the delegation, told RIA news agency "Russian aviation will help the Syrian army's ground offensive operation".

The deployment of the Russian air force to Syria last year helped tip the war Assad's way as it bombed rebels supported by his enemies including Saudi Arabia, Turkey and the United States. 

President Vladimir Putin last month withdrew some of the Russian forces, but maintained an air base in Latakia, and kept up strikes on the Islamic State group.

Neither the al Qaeda-linked Nusra Front or Islamic State are included in the partial ceasefire.