London (AFP) - The US and France have accused President Bashar al-Assad's regime of undermining talks to end Syria's civil war, which were suspended hours before donors were due to meet Thursday to raise aid for the conflict-torn country.

French French Foreign Minister Laurent Fabius accused Damascus and Russia of "torpedoing the peace efforts" by launching an offensive near Aleppo, and said world powers would hold "in-depth consultations" on their actions at the conference.

The UN paused the fruitless peace negotiations on Wednesday as the Syrian government said it had cut a key supply route to Syria's second city from the Turkish border with the help of Russian air strikes.

It is one of several regime offensives since Moscow began bombing in September, and Russian Foreign Minister Sergei Lavrov said Wednesday he saw no reason to stop until the "terrorists" are defeated.

The talks had been tipped as the most important push so far to end Syria's bloody five-year conflict, which has killed more than 260,000 people and forced half the country's population from its homes.

But they stalled before any progress had been made, with the main opposition umbrella group saying it would not return to Geneva until the regime agreed to end bombardments and allow aid into besieged cities.

UN special envoy Staffan de Mistura said discussions would resume on February 25, insisting this was "not the end or the failure of the talks" and calling on all players to work harder to make them a success.

US Secretary of State John Kerry said the Syrian government's push to grab territory as negotiations were meant to be getting underway proved the regime was not serious about the talks.

"The continued assault by Syrian regime forces -- enabled by Russian air strikes -- against opposition-held areas... have clearly signalled the intention to seek a military solution rather than enable a political one," he said.

- 'Biggest humanitarian crisis' -

The suspension of the talks came as donors were due to gather in London with the aim of raising $9 billion in aid for Syria and to help its neighbours cope with millions of people that have taken refuge on their soil.

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Ahead of the start, Britain pledged £1.2 billion (1.6 million euros, $1.74 billion) in aid, to be spent between 2016 and 2020, to address what Prime Minister David Cameron called "the world's biggest humanitarian crisis".

Some 4.6 million Syrians have fled to nearby countries -- Jordan, Lebanon, Turkey, Iraq and Egypt -- while hundreds of thousands have journeyed to Europe in the region's biggest migration crisis since World War II.

"With hundreds of thousands of people risking their lives crossing the Aegean or the Balkans, now is the time to take a new approach to the humanitarian disaster in Syria," Cameron said.

Aid from the conference will be targeted at helping the economies of Syria's neighbours, creating jobs for refugees and citizens of their host countries. It will also go towards food, shelter, medical care and rebuilding health facilities in Syria itself.

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

Norwegian Foreign Minister Borge Brende said there was a "moral imperative, human imperative" to act.

"It's a lost generation if we're not successful tomorrow," he said ahead of the conference.

- Talks scuppered -

De Mistura's brief at the Geneva discussions was to coax both sides into six months of indirect "proximity talks" envisioned under a November roadmap, but problems plagued his efforts from the outset.

The opposition High Negotiations Committee (HNC) arrived several days late -- and reluctantly -- 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.

Riad Hijab, HNC chief coordinator, said late Wednesday the group "will not return until the humanitarian demands are met or (we) see something on the ground".

The Syrian government delegation in Geneva, meanwhile, complained that the Saudi-backed HNC was disorganised, had not named its negotiators and contained individuals it considered "terrorists".

One such figure is Mohammed Alloush, a leading member of Islamist rebel group the Army of Islam and nominally the HNC's chief negotiator, who arrived in Geneva late on Monday.

Bashar al-Jaafari, chief government negotiator, blamed the suspension on opposition "preconditions" and said de Mistura announced the break only because the HNC was about to leave.