He noted that even in the U.S., "there are experts who believe that the evidence presented by the administration doesn't look convincing, and they don't exclude the possibility that the opposition conducted a premeditated, provocative action trying to give their sponsors a pretext for military intervention."
He compared the evidence presented by Washington to false data used by the Bush administration about weapons of mass destruction 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 U.S. called a mistake. Did we forget about that?" Putin said.
He said he "doesn't exclude" backing the use of force against Syria at the United Nations if there is objective evidence proving that Assad's regime used chemical weapons against its people. But he strongly warned Washington against launching military action without U.N. approval, saying it would represent an aggression. Russia can veto resolutions at the U.N. Security Council and has protected Syria from punitive actions there before.
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 said it is premature to talk about what Russia would do if the U.S. attacked Syria.
"We have our own ideas about what we would do and how we would do it if the situation develops toward the use of force or otherwise," he said. "We have our plans, but it's too early to talk about them."
Putin called the S-300 air defense missile system "a very efficient weapon" and said that Russia had a contract for its delivery of the S-300s to Syria. "We have supplied some of the components, but the delivery hasn't been completed. We have suspended it for now," he said.
"But if we see that steps are taken that violate the existing international norms, we shall think how we should act in the future, in particular regarding supplies of such sensitive weapons to certain regions of the world."
The statement could be a veiled threat to revive a contract for the delivery of the S-300s to Iran, which Russia canceled a few years ago under strong U.S. and Israeli pressure.
Putin praised Obama as a frank and constructive negotiating partner and denied reports that he had taken personal offense at remarks by Obama comparing Putin's body language to that of a slouching, bored student. Putin said appearances can be deceiving.
Putin also accused U.S. intelligence agencies of bungling efforts to apprehend Snowden, the National Security Agency leaker, who is wanted in the U.S. on espionage charges. He said the United States could have allowed Snowden to go to a country where his security would not be guaranteed or intercepted him along the way, but instead pressured other countries not to accept him or even to allow a plane carrying him to cross their airspace. Russia has granted him temporary asylum.
Putin also gave the first official confirmation that Snowden had been in touch with Russian officials in Hong Kong before flying to Moscow on June 23, but said he learned that Snowden was on the flight only two hours before it arrived. Putin once again denied that Russia's security services are working with Snowden, whose stay in Russia has been shrouded in secrecy.
On another topic, he denied at length charges that Russia has anti-gay policies, indicating that Obama is welcome to meet with gay and lesbian activists in Russia during his visit. He said he might even meet with a similar group himself if there is interest from the Russian gay community.
Putin rejected criticism of the gay propaganda law, which prompted some activists to call for the boycott of the 2014 Winter Games in Sochi, arguing that it won't infringe on the rights of gays.
He also said that athletes and activists will not be punished if they raise rainbow flags or paint their fingernails in rainbow colors at the Feb. 7-23 Olympics.