* Tough Syria talks with Putin dominate G8 summit
* Putin defiant, weighs arms sales to Syria
* Obama, Putin spar over Syria
* G8 calls for political solution, avoids mention of Assad (Recasts, adds new elements, context)
By Andrew Osborn and Maria Golovnina
ENNISKILLEN, Northern Ireland, June 18 (Reuters) - Russia's Vladimir Putin derailed Barack Obama's efforts to win backing for the downfall of Syrian leader Bashar al-Assad at a G8 summit on Tuesday, warning the West that arms supplied to the rebels could be used for attacks on European soil.
After two days of intense talks that fell far short of what Obama and Prime Minister David Cameron had been hoping for, Putin fumed against Western moves to supply weapons to rebels while defending his own supplies of arms for Assad.
"We are supplying weapons under legal contracts to the legal government. That is the government of President Assad. And if we are going to sign such contracts, we are going to deliver," the Russian president said.
Putin, isolated at the summit, repeatedly clashed with other leaders over the fate of Assad and resisted pressure to agree to anything that would imply Assad should step down. In the end, a G8 communique did not even mention Assad's name.
The summit in a secluded golf resort in Northern Ireland ended with G8 leaders calling for peace talks to be held as soon as possible to resolve the Syrian civil war. This has broadly been their position for months.
No date was mentioned for a peace conference called by Moscow and Washington, which was supposed to take place next month but now appears to be on hold, after the United States announced last week that it would arm the rebels.
A source at the summit said the peace conference would now be put off at least until August.
Putin struck a defiant tone: he hinted that Obama had tried to isolate Russia, that other leaders were divided, and that plans to send arms to Syrian rebels could lead to murders such as that of a British soldier on a busy London street last month.
"British people have lately witnessed a tragedy, and we lived through it together, when right in the streets of London a British army serviceman was brutally murdered outside his barracks," Putin said.
"Is it these people that the Europeans want to supply arms? What happens next with those weapons? Who will control in which hands they end up? They could possibly (end up) in Europe."
Obama and his allies want Assad to cede power while Putin, whose rhetoric has become increasingly anti-Western since he was re-elected last year, believes that would be disastrous at a time when no clear transition plan exists.
Russia has been Assad's most powerful supporter shielding the Syrian leader from Western action as his forces struggle to crush an uprising in which 93,000 people have been killed since March 2011 and which is now drawing in neighbouring countries.
It has vetoed United Nations Security Council resolutions censuring the Assad government, widely criticised for the ferocity with which it has waged the war.
Syria is one of Moscow's last allies in the Middle East. Its influence has declined since the collapse of the Soviet Union but the Russian navy still has a base at the Mediterranean port of Tartus.
The United States and its European and Gulf Arab allies have repeatedly called on Assad to surrender power and predicted his downfall. Recent battlefield gains by government forces against the rebels make that prospect unlikely anytime soon.
In the final document, G8 leaders called on the Syrian authorities and the opposition to commit to destroying all organisations affiliated with al Qaeda - a reflection of growing concern in the West that Islamist militants were playing a more dominant role in the rebel ranks.
British Prime Minister David Cameron, who chaired the summit, said separately after the talks that the West believed strongly that there was no place for Assad in a future Syria.
"It is unthinkable that President Assad can play any part in the future of his country. He has blood on his hands," Cameron told reporters at a podium perched on the shore of a picturesque lough flanked by rolling hills.
"You can't imagine a Syria where this man continues to rule having done such awful things to his people."
Cameron said the main breakthrough was an agreement that a transitional government with executive powers was needed and a deal to call for an investigation into chemical weapons use.
Both, however, are old positions that have already been agreed. The West and Russia still disagree over whether Assad should be excluded from the transitional government, and over how to carry out chemical weapons investigations.
"We remain committed to achieving a political solution to the crisis based on a vision for a united, inclusive and democratic Syria," the final communique read.
"We strongly endorse the decision to hold as soon as possible the Geneva conference on Syria," it said, without saying when the conference should be held.
For his part, Putin renewed criticism of U.S. plans to send weapons to Syrian rebels, which the Obama administration announced after concluding that Assad's forces had used nerve gas. Putin said other G8 leaders had expressed doubts that Assad's forces had used chemical weapons.
"Let me assure you that not all G8 members believe it was used by the Syrian army. Some agree with our opinion that there's no such data," Putin said.
During the talks, Western powers faced strong resistance from Putin as they tried to hash out a statement with teeth that all G8 leaders could agree on.
Looking mostly tense throughout the meeting, Putin had faced a barrage of criticism over his Syria stance. Canadian Prime Minister Stephen Harper accused him of supporting "thugs" in Damascus, while his meeting with Obama was frosty and both looked uncomfortable.
Russia's position is that only Syrians can decide Assad's fate. The West considers that to be cover for allowing him to stay in power. Russian Deputy Foreign Minister Sergei Ryabkov, speaking on the sidelines, said any debate about Assad's role in the resolution of the conflict was unthinkable.
"This would be not just unacceptable for the Russian side, but we are convinced that it would be utterly wrong, harmful and would completely upset the political balance," Ryabkov said. (Additional reporting by Andrew Osborn, William Schomberg, Roberta Rampton, Alexei Anishchuk, Jeff Mason and Kate Holton in Enniskillen; Writing by Maria Golovnina and Guy Faulconbridge; Editing by Peter Graff)