London (AFP) - The United States demanded Thursday that Russia immediately halt its bombing campaign in Syria after a bitter breakdown in peace talks exposed the deep rift between world powers aiming to end the five-year conflict.

On the ground, nearly 40,000 people have fled an offensive this week by President Bashar al-Assad's regime north of the city of Aleppo, according to the Syrian Observatory for Human Rights monitor.

Assad's forces also entered two Shiite villages that were under siege by rebels, prompting what state news agency SANA called "mass celebrations" in the streets of Nubol and Zahraa.

International donors were meeting in London on the Syria crisis just hours after the peace talks in Geneva were suspended Wednesday until February 25, with UN special envoy Staffan de Mistura saying "more work" was needed.

The talks had been tipped as the most important push so far to end Syria's brutal conflict, which has killed more than 260,000 people and forced half the country's people from their homes since March 2011.

Russia has been supporting the Assad regime with air strikes since September which it says are targeted at "terrorist organisations" such as the Islamic State group.

US Secretary of State John Kerry warned Moscow to halt its bombing of the Syrian opposition in what he said was a "robust" phone call with Russian counterpart Sergei Lavrov.

"We discussed, and we agreed, that we need to discuss how to implement the ceasefire," he said, adding he and Lavrov would speak again later Thursday or on Friday.

The Russian foreign ministry said Kerry and Lavrov had agreed to do everything possible to make the break in peace talks "as short as possible."

French Foreign Minister Laurent Fabius accused Syria and Russia of "torpedoing the peace efforts" with the offensive.

UN Secretary-General Ban Ki-moon said the pause in the talks showed "just how deep, how difficult the divisions are".

- 'Eating grass to survive' -

The suspension of the talks came as donors gathered in the British capital aiming to raise billions of dollars in aid for Syria and to help its neighbours cope with millions of people who have taken refuge on their soil.

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Co-host British Prime Minister David Cameron urged a political transition away from Assad "however difficult that may be".

Britain pledged £1.2 billion (1.6 million euros, $1.74 billion) to be spent between 2016 and 2020 on what Cameron called "the world's biggest humanitarian crisis".

German Chancellor Angela Merkel, under growing pressure over her open door policy for refugees amid Europe's biggest such crisis since World War II, pledged 2.3 billion euros.

And Kerry announced $890 million from the United States.

"If people are reduced to eating grass and leaves and killing stray animals in order to survive, that's something that should tear at the conscience of all civilised people," he said.

Some 4.6 million Syrians have fled to nearby countries -- Jordan, Lebanon, Turkey, Iraq and Egypt -- while hundreds of thousands have journeyed to Europe.

Jordan's King Abdullah II told the conference his country of around 6.5 million people had "reached our limit" after taking nearly 1.3 million refugees.

Lebanon's Education Minister Elias Bou Saab told BBC radio that his nation of four million people had taken 1.5 million Syrian refugees and was dealing with an "earthquake".

Up to 70,000 more people from camps near Aleppo are now moving towards Turkey as a result of the recent aerial bombardments, according to Turkish Prime Minister Ahmet Davutoglu.

"My mind is not now in London but on our border (and) how to relocate these new people coming from Syria," he told the conference.

As well as drumming up aid, the meeting aims to allow more refugees to work in their host countries and boost their education.

The UN is appealing for nearly $8 billion, while regional governments are seeking an extra $1.2 billion.

Organisers have already agreed that participants should at least "double" their contributions from 2015, when they raised $3.3 billion.

- Problems from the start -

The Geneva initiative aimed to coax both sides into six months of indirect "proximity talks" under a November roadmap but there were problems from the start.

The opposition High Negotiations Committee (HNC) arrived reluctantly and several days late, insisting on immediate steps to allow aid to get through to besieged cities, a halt to the bombardment of civilians and the release of thousands of prisoners.

Munzer Makhous from the HNC told AFP on Thursday that the pause in the talks was "the right decision" and called on the United States to help create "a balance" between the opposing delegations to avoid failure in a fresh round.

"There needs to be pressure from the Americans on the Russians," he said in Geneva, hours before the HNC delegation was due to leave the Swiss city.

"The balance needs to change, at least so it can be equal between the government and the opposition so each side can then make compromises," he added.