A 48-hour truce took hold in Syria's war-torn Aleppo city on Thursday as key regime backer Russia blamed the United States for the lack of progress in peace talks.

The local ceasefire came hours after US Secretary of State John Kerry warned Moscow that Washington's patience was running out over breaches of a nationwide truce.

Aleppo has seen some of the worst fighting in a war that has killed more than 280,000 people, but there is deep scepticism that the latest halt to fighting in the northern city will last.

Peace talks aimed at ending the five-year conflict have stalled and the February 27 countrywide ceasefire between the regime and non-jihadist rebels lies in tatters.

"There is no progress in the political process," Russian Foreign Minister Sergei Lavrov said at an economic forum in Saint Petersburg, referring to Syria.

He accused Washington, which supports Syrian rebels, of being "unable or unwilling to put pressure on its allies in the region".

Nevertheless, direct contact between Russia and the United States about Syria have taken place "without any hysteria", he added.

There have been repeated violations of the February 27 truce in Aleppo, with rebels pounding regime-controlled neighbourhoods with rocket and artillery fire and the regime hitting rebel areas with air strikes.

The new 48-hour ceasefire was announced by Moscow late Wednesday in a bid to halt violence in the city, split since 2012 between a regime-held west and a rebel-controlled east.

"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 stabilising the situation," Russian's defence ministry said.

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

Residents in Aleppo's rebel-held east headed out to the markets to buy meat and vegetables on Thursday morning, an AFP reporter there said.

After weeks of air strikes, it was the first time they have shopped in the morning since the start of the holy Muslim month of Ramadan.

Syria's 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 President Bashar al-Assad.

But a Syria analyst, Karim Bitar, described the latest local truce in Aleppo as a "ceasefire of convenience, which is not linked to a real political process".

"Syrians are increasingly sceptical about these brief ceasefires, which seem to have become as artificial and fruitless and the negotiation sessions -- which each time revive hopes then end in bitter disappointment."

The Britain-based Syrian Observatory for Human Rights -- which relies on sources inside Syria to gather information on the war -- agreed.

"This temporary truce of a few hours doesn't aim to end the bloodshed, but to give some rest to the killers -- those who massacre the people of Aleppo and Syria -- before they resume their crimes," the Observatory said in a statement.

UN-backed peace talks in Geneva came to a deadlock in April after the opposition walked out over increasing violence and lack of humanitarian access to besieged areas.

The United Nations says nearly 600,000 Syrians live in besieged areas, most surrounded by government forces.

Dozens of opposition activist groups, in a report authored by The Syria Campaign advocacy group, on Wednesday accused the UN of "capitulating" to Damascus on aid access.

The report -- which was based on testimonies from current and former UN staff and other aid workers -- accused the UN of "choosing to prioritise cooperation with the Syrian government at all costs," allowing the regime to unduly influence UN aid strategy.

The UN humanitarian coordinator for Syria, Yacoub El Hillo, said however that while aid access was not ideal, the UN continues to "assist Syrians based on need".