President Vladimir Putin warned the West against taking one-sided action in Syria but also said Russia “doesn’t exclude” supporting a U.N. resolution on punitive military strikes if it is proved that Damascus used poison gas on its own people.

In a wide-ranging interview, Putin said Moscow has provided some components of the S-300 air defense missile system to Syria but has frozen further shipments. He suggested that Russia may sell the potent missile systems elsewhere if Western nations attack Syria without U.N. Security Council backing.

Putin said he was sorry that President Barack Obama canceled a one-on-one meeting in Moscow that would have happened before the summit of G-20 nations in St. Petersburg, which opens today. But he expressed hope that the two would have serious discussions about Syria and other issues.

“We work, we argue about some issues. We are human. Sometimes one of us gets vexed. But I would like to repeat once again that global mutual interests form a good basis for finding a joint solution to our problems.”

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Obama, speaking Wednesday during a trip to Sweden that replaced his Moscow plans, said relations with Russia have “hit a wall,” but also expressed confidence the two can work together on some issues.

“I have not written off the idea that the United States and Russia are going to continue to have common interests, even as we have some very profound differences on some other issues,” he said, noting that those differences include Syria.

Putin said it was “ludicrous” that the government of Syrian President Bashar Assad — a staunch ally of Russia — would use chemical weapons at a time when it was holding sway against the rebels.

“From our viewpoint, it seems absolutely absurd that the armed forces — the regular armed forces, which are on the offensive today and in some areas have encircled the so-called rebels and are finishing them off — that in these conditions they would start using forbidden chemical weapons while realizing quite well that it could serve as a pretext for applying sanctions against them, including the use of force,” he said.

The Obama administration says 1,429 people died in the Aug. 21 attack in a Damascus suburb. Casualty estimates by other groups are far lower, and Assad’s government blames the episode on rebels trying to overthrow him. A U.N. inspection team is awaiting lab results on tissue and soil samples it collected while in Syria before completing a report.

Obama expressed frustration at Russia’s position, saying: “It has been very difficult to get Russia, working through the Security Council, to acknowledge some of the terrible behavior of the Assad regime.”

Putin, however, said the U.S. has failed to make its case through the proper channels.

“If there is evidence that chemical weapons have been used, and used specifically by the regular army, this evidence should be submitted to the U.N. Security Council,” said Putin, a former officer in the Soviet KGB. “And it ought to be convincing. It shouldn’t be based on some rumors and information obtained by intelligence agencies through some kind of eavesdropping, some conversations and things like that.”

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.

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.

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