President Vladimir Putin has said Russia could agree to back military action in Syria if the UN received proof that Bashar al-Assad's regime had used chemical weapons against its citizens.

Mr Putin said that he “doesn't exclude” the possibility of backing force, but at this stage does not even accept that an alleged chemical weapons attack took place.

And as one of Syria’s key allies on the ongoing international debate around the conflict, the Russian president warned the US that to act without the approval of the UN Security Council would be a display of “aggression”.

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Russia has suspended the delivery of missile parts to Damascus, according to the Times, but could resume it if the West takes military action in Syria without authorisation from the UN Security Council.

Moscow is sending three more ships to the east Mediterranean to bolster its fleet there. Russia is sending two destroyers, including the Nastoichivy, the flagship of the Baltic Fleet, and the Moskva missile cruiser to the region, Interfax reported today, citing an unidentified Navy official.

A US fleet is waiting for Congress to approve orders from US President Barack Obama to launch punitive strikes against Mr Assad following his forces' alleged use of poison gas.

Speaking in an interview with the Associated Press, Mr Putin said: “If there are data that the chemical weapons have been used, and used specifically by the regular army, this evidence should be submitted to the UN Security Council.

“And it ought to be convincing,” he added. “It shouldn't be based on some rumours and information obtained by special services through some kind of eavesdropping, some conversations and things like that.”

Asked what kind of evidence on chemical weapons use would convince Russia, Mr Putin said “it should be a deep and specific probe containing evidence that would be obvious and prove beyond doubt who did it and what means were used.”

Though comments about not ruling out action will be seen as a reconciliatory move ahead of the G20 summit, Mr Putin added that he thought even the idea the Assad regime could have used sarin nerve agents or other chemical weapons was “ludicrous”.

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He said: “From our viewpoint, it seems absolutely absurd that the armed forces, the regular armed forces, which are on the offensive today and in some areas have encircled the so-called rebels and are finishing them off, that in these conditions they would start using forbidden chemical weapons while realizing quite well that it could serve as a pretext for applying sanctions against them, including the use of force.”

In spite of this hint towards continued backing of the Syrian government forces, Russia has now frozen further missile defence system shipments to the country in crisis, Mr Putin said.

He admitted Russia has previously provided the Assad regime with some components of the S-300 air defence system, but said: “The whole delivery is not finalised; we have suspended it for now.”

In a veiled threat, he hinted at completing the delivery if “international norms” were broken by an attack on Syria without UN backing. Western governments fear the S-300 system could be used to shoot down their planes.

Mr Putin said it was “too early” to talk about what Russia would do if the US attacked Syria.

Nonetheless, the overall tone of his comments paves the way for one on one talks with Mr Obama this week.

Planned discussion between the pair were derailed after Russia granted asylum to US fugitive Edward Snowden, but Mr Putin told reporters: “I'm sure that even if we hold a meeting... on the sidelines of the summit, it will be useful in itself. In any case, we have many issues that we have been working on and we are interested in settling them.”

The interview with the Russian president comes as in the US Congress builds up to a vote on whether or not to authorise military action with or without UN Security Council sanction.

The Senate Foreign Relations Committee could provide its backing for the use of force as early as today.

In Britain, a spokesman for Prime Minister David Cameron gave Downing Street's reaction to Mr Putin's comments.

He said: “If members of the international community are willing to play their full part in shouldering their responsibilities, that would be welcome. We continue to engage with all G8 and G20 partners.”

Asked if Mr Cameron believes the Russian president's remarks indicate a greater willingness to contemplate approving action against Mr Assad by the UN Security Council, the spokesman said: “From the reports I have seen today, we clearly remain in very, very different places around the responsibility for the chemical weapon attacks.

“We are in very different places, but we will continue to engage with all our partners. We have been working with international partners, including Russia, for some time.”