US President Barack Obama warned Tuesday that Russia would be to blame for keeping aid from desperate Syrian civilians if it blocked a UN resolution designed to lift the siege of Homs.

Obama also heaped pressure on the Kremlin as US concern grows about the pace of Syrian compliance with a deal to hand over its chemical weapons stocks for destruction.

Obama's tough line came with President Vladimir Putin's government focused on the Winter Olympics in Sochi, but fighting its corner, and that of its ally Syria, at the United Nations.

He branded the Kremlin as a "holdout" against a Security Council resolution which would allow the delivery of food, shelter, medical aid and water to Homs and other cities where thousands of civilians are trapped by fighting.

"There is great unanimity among most of the Security Council on this resolution," Obama said.

He said Secretary of State John Kerry had told Russia that "they cannot say that they are concerned about the well-being of the Syrian people when they are starving civilians."

"It is not just the Syrians that are responsible, the Russians (are) as well if they are blocking this kind of resolution," Obama said at a White House press conference with French President Francois Hollande.

Western states want Russia to back a draft resolution which calls on all parties to "immediately end the sieges of the Old City of Homs" and other Syrian cities.

Russian Foreign Minister Sergei Lavrov said though that the draft was "absolutely unacceptable," because it contained an ultimatum for the government of President Bashar al-Assad.

"Instead of engaging in everyday, meticulous work to resolve problems that block deliveries of humanitarian aid, they see a new resolution as some kind of simplistic solution," he said.

The resolution is not binding and does not provide for automatic sanctions.

But if its demands were not enacted within 15 days, the Council could vote for sanctions targeting those who blocked delivery of humanitarian aid or harmed civilians.

Obama also laid down a warning to Russia over the chemical weapons deal agreed between Moscow and Russia to avert US strikes on Syria last year.

Washington has said only limited shipments of chemicals have so far left the Syrian port of Latakia -- far fewer than the 700 tonnes Damascus was to hand over by the end of 2013, under a US-Russia brokered agreement.

"Syria must meet its commitments and Russia has a responsibility to ensure that Syria complies," Obama said.

Russian Ambassador to the United Nations Vitaly Churkin said last week that despite rising US reservations, things were "moving along" with the chemical weapons deal.

Obama admitted that Syria had indeed missed some deadlines, as his political opponents warn the sluggish pace of disposal exposes the weapons deal as unworkable.

Yet Obama said that the agreement had allowed for the cataloguing of all chemical weapons stocks in Syria.

The UN Security Council last year backed the US-Russian deal to destroy Syria's vast chemical arsenal as a way to avert US strikes threatened after chemical attacks near Damascus that Washington blamed on the regime.

Under the agreement, Syria's entire chemical arsenal is due to be eliminated by June 30.

Western powers accuse President Bashar al-Assad's regime of purposefully delaying the operations, while Syria stresses the challenges it faces in meeting its commitments during a time of war.

Following widespread commentary that talks in Geneva to bring government and opposition supporters together to search for an end of a civil war was a failure -- Obama chose to see the merest of silver linings.

"I would not completely discount the fact that in this latest round of negotiations, what you saw was a coherent, cohesive, reasonable opposition in the same room for the first time negotiating directly with the regime.

"Now, the regime, Assad's regime, wasn't particularly responsive, and I think even some of their patrons were disturbed by their belligerence."