MUNICH: World powers on Friday agreed on an ambitious plan to cease hostilities in war-wracked Syria within a week and dramatically ramp up humanitarian access at talks in Munich aimed at ending the five-year war.

The 17 countries agreed "to implement a nationwide cessation of hostilities to begin in a target of one week's time," said United States (US) Secretary of State John Kerry after extended talks co-hosted by Russian Foreign Minister Sergei Lavrov.

The International Syria Support Group also agreed "to accelerate and expand the delivery of humanitarian aid beginning immediately".

"Sustained delivery will begin this week, first to the areas where it is most urgently needed and then to all the people in need throughout the country, particularly in the besieged and hard to reach areas," said Kerry.

An onslaught on the key rebel stronghold of Aleppo by Syrian government troops, backed by Russian bombers and Iranian fighters, derailed peace talks this month and forced 50,000 people to flee.

The bombardments have left the opposition virtually encircled and observers say 500 people have died since they began on February 1, the latest hellish twist in a war that has claimed more than 260,000 lives.

Kerry said talks between rebels and Syrian President Bashar al-Assad's regime would resume as soon as possible, but warned that "what we have here are words on paper".

"What we need to see in the next few days are actions on the ground," he said.

Host German Foreign Minister Frank-Walter Steinmeier agreed, adding that "whether this really is a breakthrough we will see in the next few days".

"When the whole world sees whether today's agreements are kept and implemented by the Assad regime and the Syrian opposition, by Hezbollah and opposition militias, and also by Russia," he said.

Fate of Assad

The atmosphere going into the talks had been gloomy, with Russian Prime Minister Dmitry Medvedev even warning of a "new world war" if Gulf nations sent in troops to support the rebel opposition.

But the working group emerged with a document that showed a surprising level of cooperation between the key players, despite rising tensions over Moscow's bombing campaign.

Lavrov called "for direct contacts between the Russian and US military" in Syria.

Kerry said the cessation of hostilities, an intentionally more tentative phrasing than a full ceasefire would apply to all groups apart from "the terrorist organisations" of the militant Islamic State (IS) group and Al Qaeda affiliate Al Nusra.

But Russia and the US remain starkly at odds on several issues, particularly the fate of Assad.

The two traded accusations on Thursday, with the Pentagon claiming two Russian air strikes had destroyed hospitals in Aleppo and denying Russian claims that US planes had struck the city.

Syria is a crucial ally and military staging post for Russia and Iran, while observers say Moscow has benefited from the chaos created by the war, particularly the refugee crisis it has created in Europe.

Washington, reluctant to involve itself in another complex war after the quagmires of Afghanistan and Iraq, has also faced criticism for not doing enough to help the rebels.

Instead, it has sought to focus more on combatting IS that has taken over swathes of Syria and Iraq, than getting involved in the civil war between the regime and opposition forces.

"The US has given up the idea of toppling Assad," said Camille Grand, of the Foundation for Strategic Research in Paris. "Kerry seems willing to accept pretty much anything to resolve the crisis."

The conflict has also strained relations between Turkey and its Western allies.

Turkish President Recep Tayyip Erdogan has slammed Washington's increasingly close alliance with the Kurdish militias in the fight against IS, saying it was turning the region into "a pool of blood".