President Vladimir Putin Wednesday suggested Russia could approve military strikes against the Syrian regime if the West presented watertight evidence of chemical weapons crimes but warned the use of force without UN approval would be an "aggression".
In an interview with state-run Channel One television ahead of the G20 summit in Saint Petersburg this week, Putin sought to adopt a more conciliatory position on the Syria crisis which had widened the rift between Russia and the West.
Yet he still appeared unconvinced by US assertions that the Syrian regime was behind a chemical attack outside Damascus on August 21 that left hundreds dead and said the West had to present "convincing" evidence to the UN Security Council.
Asked whether Russia would agree with US-led military strikes if it was proven that the Syrian regime had carried out the chemical attack, Putin replied: "I do not exclude that."
But he said it would be unacceptable for the West to go ahead with military action against the regime of Bashar al-Assad without the assent of the UN Security Council, where Russia has veto-wielding permanent membership.
"Only the UN Security Council can give approval for the use of force against another state," Putin said.
"Any other ways to justify the use of force against another sovereign and independent state are unacceptable and cannot be qualified as anything other than aggression," Putin said.
"When questions linked to the use of force are decided outside the UN Security Council then the danger appears that such illegal decisions can be used against anyone or under any pretext," he added.
The United States has indicated it is prepared to go ahead with military action without UN approval, but President Barack Obama is first seeking approval from Congress which will push back the timetable until after the G20 which begins Thursday.
Putin said the West still needed to put forward cast-iron proof of the circumstances of the attack, which some Russian officials have previously blamed on rebels seeking to discredit the regime.
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"If there is evidence that chemical weapons were used, and by the regular army,... then this evidence must be presented to the UN Security Council. And it must be convincing," Putin said.
"It cannot be based on rumours, information based on special services' eavesdropping or chatter and so forth," Putin said.
If there was clear proof of what weapons were used and who used them, Russia "will be ready to act in the most decisive and serious way," Putin said.
But he reaffirmed his past arguments it would have been "absurd and not in line with any logic" for the Syrian regime to have used chemical weapons at a time when it is on a military offensive.
Putin appeared to acknowledge he had seen the "horrific images" posted on the Internet that allegedly show children killed in the chemical attack outside Damascus.
"But the question is, who did what and who is guilty," Putin said, saying that some believed that the pictures were a montage by the radical Islamist rebels fighting Assad.
Putin confirmed that Russia had delivered some components of sophisticated S-300 missile systems to Syria but revealed for the first time deliveries had now been halted.
"We have delivered separate components but the whole delivery has not been completed and for the moment we have suspended it," Putin said, without specifying the reason for halting the deliveries.
The contract had been vehemently condemned by the West which argued that Moscow was handing Damascus the firepower to hit back at eventual air attacks.
A military source told the state RIA Novosti news agency after the interview was broadcast that the components delivered by Moscow to Damascus were thus far not enough to mount a single complete S-300 missile system and could not be used as a weapon.