The Russian air force and Syrian military are preparing a joint operation to take Aleppo from rebels, the Syrian prime minister was quoted saying on Sunday, April 10 and an opposition official said a ceasefire was on the verge of collapse, Reuters reports.

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 U.S. 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.

"We, together with our Russian partners, are preparing for an operation to liberate Aleppo and to block all illegal armed groups which have not joined or have broken the ceasefire deal," he was quoted as saying by TASS news agency.

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.

Rebels have reported the resumption of Russian air strikes south of Aleppo, an important theater where Iranian forces and Lebanon's Hezbollah are fighting in support of the army and the Nusra Front is deployed in close proximity to rebels.

The Syrian Observatory for Human Rights said a total of 35 combatants had been killed on both sides in a 24 hour period in the area, where fighting has been raging for some 10 days.