MOSCOW (Reuters) - Information that the United States has given Russia about suspected use of chemical weapons by Syrian President Bashar al-Assad's forces "does not look convincing", President Vladimir Putin's senior foreign policy adviser said on Friday.
Yuri Ushakov said more U.S. military support for Assad's opponents would undermine joint efforts to bring together Syrian government and opposition representatives for peace talks.
But he said Russia was "not yet" considering sending Assad advanced missiles in response and voiced hope for constructive talks between Putin and Western leaders at a G8 summit next week, saying Moscow and Washington do not "compete" over Syria.
A U.S. official said on Thursday that President Barack Obama had authorised sending U.S. weapons to Syrian rebels for the first time, after the White House said it had proof the Syrian government had used chemical weapons against opposition forces.
U.S. officials briefed Russia on the information they had, Ushakov said. "But I will say frankly that what was presented by the Americans does not look convincing to us."
Syria's Foreign Ministry accused the United States of lying about chemical weapons use to give it an excuse to intervene, saying it relied on "fabricated information".
Ushakov made clear that Russia suspects there could be a repeat of the run-up to the U.S.-led invasion of Iraq in 2003, when false intelligence was used to justify military action.
"I would not like to draw parallels with the well-known investigations by (then U.S.) Secretary of State (Colin) Powell, but the information and facts that were presented do not look convincing," he said of the latest U.S. information.
"It would be hard even to call them facts," Ushakov said at a briefing before the June 17-18 G8 summit in Northern Ireland, where Putin will meet Obama and other Western leaders.
No date has been set for a conference Russia and the United States are trying to convene to bring together those fighting in a two-year-old civil war that has killed 93,000 people.
"If the Americans ... carry out more wide-scale aid to the rebels and opposition, it will not make organising the international conference easier," Ushakov said.
Russian Foreign Ministry spokesman Alexander Lukashevich said that the U.S. plans caused "serious concern" and suggested the United States was turning toward military aid because its efforts to bring Assad's foes to the conference were "stalling".
In a statement, Lukashevich expressed concern about calls for the creation of a no-fly zone, which Russia has opposed.
Russia has rejected Western and Israeli pressure to scrap a deal to send Assad advanced S-300 air-defence missile systems, saying they would deter "hotheads" eager to intervene, but Putin said last week that the equipment had not yet been delivered.
Asked whether Moscow might send S300s to Syria as a response to increased U.S. military support for the rebels, Ushakov said: "We are not talking about that yet."
"We are not competing over Syria," Ushakov said. "We are not talking about competition at all. I think Putin and Obama will discuss the further development of Russian-American initiatives when they meet (in Northern Ireland)."
Russia has been Assad's most powerful protector during the conflict, opposing sanctions and, with China, vetoing three U.N. Security Council resolutions aimed at putting pressure on Assad's government or pushing him from power.
Moscow has been a major supplier of weapons to Syria since the Soviet era but says it is no longer delivering arms that can be used in the civil war.
(Writing by Thomas Grove and; Steve Gutterman; Editing by Louise Ireland)