Vladimir Putin, the Russian president, has warned the US against taking one-sided action in Syria, but has also said that Russia "doesn't exclude" the possibility of supporting a UN resolution authorising military strikes.
He said that such an endorsement would require "convincing" evidence that President Bashar al-Assad's government used chemical weapons against citizens.
He also said the currently available evidence does not fulfil this criteria.
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In a wide-ranging interview with the Associated Press news agency and Russia's state Channel 1 television, Putin said it would be "absolutely absurd" for Assad's forces to have used chemical weapons at a time when they were in the ascendency in the conflict.
"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 realising quite well that it could serve as a pretext for applying sanctions against them, including the use of force," Putin said in the interview, released on Wednesday.
US President Barack Obama said in Stockholm on Wednesday that the international community "cannot be silent" following Syria's alleged use of chemical weapons against its own people.
"I discussed our assessment and (Swedish Prime Minister Fredrik Reinfeldt) and I are in an agreement that in the face of such barbarism the international community cannot be silent," he said.
Figures vary regarding the alleged chemical weapons attack on August 21, with the US government saying that 1,429 people were killed by poison gas in the attack, and aid agencies putting that number at closer to 355.
Assad's government has blamed the attack on the rebels, and a UN inspection team that examined the attack sites near Damascus is awaiting lab results on soil and tissue samples.
"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,'' Putin added in his interview.
"And it ought to be convincing. 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."
He also cited experts who believed that the current evidence "doesn't look convincing", and raised the possibility that the armed opposition had "conducted a premeditated provocative action trying to give their sponsors a pretext for military intervention".
Putin compared the evidence presented by the US administration so far to false data used by that country to justify the 2003 invasion of Iraq.
"All these arguments turned out to be untenable, but they were used to launch a military action, which many in the US called a mistake. Did we forget about that?" he said.
UN action not excluded
In the interview, Putin did not "exclude" the possibility of the use of force by foreign countries against Syria, if there was enough evidence provided to the UN and that body sanctioned such an action.
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.
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He strongly cautioned the US against launching military action without UN approval, however, saying it would represent an aggression.
Asked what kind of evidence on chemical weapons use would convince Russia, 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".
Putin also said Russia has provided some components of the S-300 air defence missile system to Syria but that the delivery had not been completed.
He said that the process remained suspended "for now".
The interview on Tuesday night at Putin's country residence outside Moscow was the only one he granted prior to the summit of G-20 nations in St Petersburg, which opens on Thursday and will see major world powers discuss the global economy and the crisis in Syria.
US considers Syria action
Meanwhile, US national security officials will on Wednesday hold a series of public and private hearings with members of the US House of Representatives regarding the authorisation of the use of military force against Syria.
The meetings come after leaders of the US Senate Foreign Relations Committee said on Tuesday that they had reached an agreement on a draft authorisation, paving the way for a vote by the committee on Wednesday.
Among other provisions, the draft, which was obtained by Al Jazeera, sets a 60-day limit on military action in Syria, with a possibility for a single 30-day extension subject to conditions.
The deal reached by Senator Robert Menendez, the Democratic chairman of the panel, and Senator Bob Corker, the top Republican, includes a provision banning any use of US armed forces on the ground.
If the document is approved by the committee on Wednesday, it will then be sent to the full Senate for a vote after members return from their August recess on September 9.
Earlier John Kerry, US secretary of state, and Chuck Hagel, the defence secretary, gave evidence to the committee, where they assured the committee that there was "zero intention" of putting troops on the ground.
The draft resolution requires Obama to consult Congress and submit to the Senate and House of Representatives foreign relations panel a strategy for negotiating a political settlement to the Syria conflict, including a review of all forms of assistance to the rebels fighting to oust President Bashar al-Assad.
Menendez said that the authorisation remained "narrow and focused" and "limited in time".
In a crucial show of support on Tuesday for Obama, John Boehner, House speaker and a Republican in Congress, said he would back Obama's motion for strikes against Assad, and called on his party colleagues to do the same.
His Republican colleague, House majority leader Eric Cantor, also supported Obama's call.
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Source: Al Jazeera and agencies