By Thomas Grove and Yara Bayoumy
MOSCOW/BEIRUT, Sept 4 (Reuters) - President Vladimir Putin offered a glimpse of international compromise over Syria on Wednesday, declining to entirely rule out Russian backing for military action as he prepared to host a summit of world leaders.
As the United States and allies prepare to bypass any Russian U.N. veto and attack Damascus, there is little chance of Putin's support. But his words may herald new efforts to overcome great power rivalries that have let Syria descend into bloody chaos.
At the same time, Moscow said it had sent a warship it calls a "carrier killer" to the eastern Mediterranean, where a U.S. fleet is waiting for Congress to approve orders from President Barack Obama to launch punitive strikes against Syrian President Bashar al-Assad following his forces' alleged use of poison gas.
Putin's remarks on the eve of a G20 summit in St. Petersburg stressed Russia did not share Obama's conviction that Assad has resorted to chemical warfare - he noted suggestions the Aug. 21 gassing was instead the work of al Qaeda-linked rebels.
And only proof, plus backing in the U.N. Security Council that depends on Moscow, would justify using force, he added. Nonetheless, in saying he did "not rule out" his support, Putin gave a shot of warmth to relations with the West that the Syrian conflict has helped chill to levels recalling the Cold War.
Moscow has been the main arms supplier to Assad, who is also backed by Iran as part of Tehran's wider confrontation with the United States and its allies in Israel and the Gulf Arab states.
And Russian media reports on military deployments have provided a reminder of continuing tensions. On Tuesday, Russian reports of missile launches in the Mediterranean moved the world oil market and set nerves on edge in Damascus before Israel explained it had fired a rocket in exercises with U.S. forces.
On Wednesday, Russia's Interfax news agency quoted a source saying the guided missile cruiser Moskva was heading to the eastern Mediterranean to take over as flagship. The agency noted that the Soviet-era Moskva, designed to attack other ships, was known as a "carrier killer". Only the United States and European allies are likely to deploy any aircraft carriers in the region.
Western officials say they do, however, detect some signs of willingness on the part of both U.S. allies and Russia to resume efforts to resolve a bloody civil war in which both sides seem entrenched and which is destabilising the entire Middle East.
A senior Western official said that, while Moscow was unlikely to say so in public, there were signs Russian officials believe Assad was indeed responsible for the chemical weapons attack and it had strained Russian support for him - providing an opening for a new, concerted drive to end the conflict.
Stalemate in the U.N. Security Council between Russia, backed by China, and the United States, backed by France and Britain, has stymied international efforts to end fighting that since 2011 has killed more than 100,000 Syrians and left millions homeless but which neither side has been able to win.
The Western official said the G20 summit, where foreign ministers will also be present to discuss Syria in particular, could provide a forum for rapprochement among the world powers.
Unease at the presence of Islamist militants in the rebel ranks - a factor Moscow has often cited in criticism of Western demands that Assad be removed forthwith - provides a point of common interest between Russia and the West.
All the major powers fear Syria descending further into anarchy. But their efforts to persuade Syrians to agree a unity government are hindered by deep hatreds fuelled by the killing and by opposing views over whether Assad should keep some power.
Following the failure of British Prime Minister David Cameron to win parliamentary backing for military strikes last week, France is the only major military power lining up behind Obama. Its parliament is to debate Syria on Wednesday, though President Francois Hollande does not need approval for action.
His foreign minister, Laurent Fabius, said using force against Assad - which will depend on U.S. congressional approval next week - could pave the way for a new round of diplomacy.
"Is an intervention a contradiction to finding a political solution?" Fabius said. "Not only is it not contradictory, but if we want a political solution, then we must help move the situation, otherwise Assad will just continue like that."
European Council President Herman Van Rompuy, who chairs the 28-nation European Union's summits, told EU ambassadors in Brussels that military action must be followed by talks.
"Calls for responsible action must include the long-term view," he said. "Only a political solution can end the terrible bloodshed, the destruction of Syria. It is time for the international community to put aside their differences and bring the parties to the conflict to the negotiations table."
Putin, who has accused Western governments of using ideas of human rights to pursue illegal wars against sovereign states, repeated his disapproval of acting without U.N. approval.
"Only the United Nations Security Council can sanction the use of force against a sovereign state. Any other approaches, means, to justify the use of force against an independent and sovereign state, are inadmissible," he told Russian television and the Associated Press.
Washington and Paris say a Russian veto in the Council should not block what they describe as a humanitarian mission to protect civilians and prevent the spread of chemical weapons.
Putin said Obama had yet to prove the case against Assad: "We have no data that those chemical substances - it is not yet clear whether it was chemical weapons or simply some harmful chemical substances - were used precisely by the official government army." There was an "opinion" they were used by rebels, some of whom are linked to al Qaeda, Putin said.
However, when asked whether Russia would agree to military action if Damascus were proven to have carried out a chemical weapons attack, he answered: "I do not rule it out."
He also said that Moscow had already sent to Syria some components of an S-300 missile system but was holding off on the delivery of final parts, something Putin threatened could happen if "existing international norms" were violated.
Western governments are concerned about the S-300 surface-to-air system, which could be used against their planes.
Regarding his relationship with Obama, Putin called the U.S. leader "a no-nonsense, practical person," and tried to dispel the idea that the pair had poor personal relations.
Obama has won the backing of key figures in the U.S. Congress, including among his Republican opponents.
Leaders of the Senate Foreign Relations Committee said they reached an agreement on a draft authorisation for the use of military force in Syria, paving the way for a vote by the committee on Wednesday. However, the draft is narrower than the request made by Obama and includes a provision barring the use of U.S. troops on the ground.
The president said on Tuesday that strikes aimed at punishing the use of chemical weapons would hurt Assad's forces while other U.S. action would bolster his opponents - though the White House has insisted it is not seeking "regime change."
Among other provisions, the Senate Foreign Relations Committee draft, which was obtained by Reuters, sets a 60-day limit on U.S. military action in Syria, with a possibility of a single 30-day extension subject to conditions.
It requires Obama to consult with Congress and submit to the Senate and House of Representatives foreign relations panel a strategy for negotiating a political settlement to the conflict, including a review of all forms of assistance to the rebels. (Additional reporting by Paul Taylor, Erika Solomon in Beirut, John Irish in Paris, Khaled Yacoub Oweis in Amman and Jeff Mason in Washington; Writing by Alastair Macdonald; Editing by Peter Graff)