Putin Russia 3Sept2013
Russian President Vladimir Putin speaks during an interview at the Novo-Ogaryovo state residence outside Moscow September 3, 2013. Reuters

Russian President Vladimir Putin Wednesday, said that he would not rule out backing a military strike on Syria, if there is solid evidence that the Syrian government used chemical weapons against its own people, but warned the U.S. that any unilateral attack without U.N. sanction would be an "aggression."

In an interview with Russian state-run Channel One television and the Associated Press, a day ahead of the G-20 summit in St. Petersburg, Putin said that he is ready to hold discussions, on the summit's sidelines, with U.S. President Barack Obama on the Syrian issue.

When asked whether Russia would support a U.N. resolution approving a military strike on Syria, Putin said: "I do not rule it out."

However, he said that Russia is still unconvinced about claims made by the U.S. that Syrian President Bashar Assad’s regime was behind the alleged chemical attack that killed more than 1,400 people in Damascus on Aug. 21. Putin said that the West will have to present watertight evidence that would connect the Syrian regime to the alleged chemical attack.

"It cannot be based on rumors, information based on special services' eavesdropping or chatter and so forth," Putin said.

“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," he said, according to a Reuters report.

Putin also made it clear that any attack without the sanction of the U.N. would be illegal. The U.S. and France had announced earlier that they might attack Syria without the U.N.’s sanction because they expect Russia to veto any resolution against the Syrian government.

However, Putin’s stance is seen as a slight deviation from his earlier position, which maintained that Russia would not back a U.N. resolution supporting a military strike on Syria.

A Western official, quoted by Reuters, said that although Russia would not publicly acknowledge it, many of its officials believed that the Syrian regime was responsible for the alleged chemical weapons attack and that this had led to a dwindling of support in Moscow for Assad.

The two-day G-20 summit, which is scheduled to begin on Thursday, is expected to focus on the Syrian crisis and Obama is expected to seek the support of international leaders for a possible military strike on Syria.