Russia announced a last minute 48-hour ceasefire for Syria's war-torn Aleppo, just hours after Washington warned Syrian President Bashar al-Assad and his ally Moscow that they must respect the nationwide truce agreed in February, AFP reports.

"On Russia's initiative, a 'regime of silence' has been introduced in Aleppo for 48 hours from 00:01 16 June (2101 GMT Wednesday) with the goal of lowering the level of armed violence and stabilizing the situation," the Russian defense ministry said in a statement.

The statement did not specify who Russia has discussed the two-day ceasefire with.

It accused Al-Nusra of attacking various Aleppo neighborhoods with multiple rocket launchers, as well as mounting a tank attack southwest of the city.

The five-year conflict has drawn in world powers who back opposing sides -- including the United States which broadly supports the opposition and Russia on the side of Assad.

"Russia needs to understand that our patience is not infinite, in fact it is very limited with whether or not Assad is going to be held accountable," said U.S. Secretary of State John Kerry on Wednesday.

"We also are prepared to hold accountable members of the opposition," he said after a meeting in Norway with his Iranian counterpart Mohammad Javad Zarif.

UN-hosted peace talks aimed at ending the conflict have been stalled since April, and a fragile ceasefire deal between the government and non-jihadist rebels has all but collapsed.