US Secretary of State John Kerry has demanded Russia stop bombing the Syrian opposition, implicitly blaming Moscow for the collapse in peace talks.

Key points: John Kerry demands Russia stop bombing Syrian opposition

John Kerry demands Russia stop bombing Syrian opposition Latest Russian-backed offensive led to collapse of UN peace talks

Latest Russian-backed offensive led to collapse of UN peace talks Turkey says talks "pointless" while Russia attacks

Speaking in London ahead of a conference on the Syrian humanitarian effort, Mr Kerry said he had called Russian Foreign Minister Sergei Lavrov for a "robust" discussion.

In Geneva on Wednesday, UN envoy Staffan de Mistura suspended attempts to begin a dialogue between Bashar al-Assad's regime and the Syrian opposition until February 25.

Alongside Britain's Foreign Minister Phillip Hammond, Mr Kerry read out sections of UN Security Council Resolution 2254, passed in December, calling for an immediate ceasefire.

"Russia has a responsibility, as do all parties, to live up to it," he said.

"So I had a conversation this morning with Foreign Minister Lavrov. We discussed, and we agreed, that we need to discuss how to implement the ceasefire."

Mr Kerry also said that both parties to the conflict — the rebels and the regime and its allies — must allow access to besieged areas for humanitarian aid.

Nearly 40,000 Syrian civilians have fled the latest regime offensive backed by Russian air strikes near Aleppo, the Syrian Observatory for Human Rights said.

The exodus began on Monday after government forces began an operation that has severed the main rebel supply route into the city and broke an opposition siege on two regime-held towns.

Turkey says talks 'pointless' while Russian attacks

Turkey's President Tayyip Erdogan said the peace talks in Geneva were "pointless" while Russia and the Assad regime continued attacks in the country.

"Russia continues to kill people in Syria. Could there be such a peace gathering? Could there be such peace talks?" Mr Erdogan said in a speech in Peru, in comments published on the presidency website.

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"In an environment where children are still being killed, such attempts do not have any function apart from making things easier for the tyrant," he said.

In London, where a donor conference for Syria is being held, Turkish Prime Minister Ahmet Davutoglu, whose government fiercely opposes the Syrian regime, also voiced concern at the huge numbers fleeing the army's offensive.

"Now 10,000 new refugees are waiting in front of the door of Kilis because of air bombardment and attacks against Aleppo," he said, referring to a Turkish town on the Syrian border.

He added that "60 to 70,000 people in the camps in north Aleppo are moving towards Turkey."

Turkey is a main backer of the Syrian opposition and has long argued that there can be no peace in Syria without Mr Assad's removal.

Mr Erdogan cast doubt on whether the talks would make meaningful progress even if they resumed.

Germany's Foreign Minister, Frank Walter Steinmeier, said the talks were difficult but not without hope.

Mr Steinmeier's comments came as Germany Chancellor Angela Merkel announced 2.3 billion euros ($3.55 billion) in aid to Syria by 2018, including 1.1 billion euros ($1.7 billion) this year alone.

The European Union said it would contribute an additional 3 billion euros ($4.65 billion) to support Syria, Jordan, Lebanon and Turkey.

The US, meanwhile, pledged $US600 million ($831 million) in humanitarian aid for Syria and $US290 million ($400 million) for neighbouring states.

AFP