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

In a wide-ranging interview with the Associated Press and Russia’s state Channel 1 television, Putin said Moscow has provided some components of the S-300 air defence missile system to Syria but has frozen further shipments.

Putin suggested that Russia may sell the potent missile systems elsewhere if western nations attack Syria without UN Security Council backing.

The interview comes ahead of the summit of G-20 nations in St. Petersburg, which opens on Thursday. The summit was supposed to concentrate on the global economy but now looks likely to be dominated by the international crisis over allegations that the Syrian government used chemical weapons in the country’s civil war.

Putin said he felt sorry that President Barack Obama cancelled a one-on-one meeting in Moscow that was supposed to have happened before the summit. But he expressed hope the two would have serious discussions about Syria and other issues.

“President Obama hasn’t been elected by the American people in order to be pleasant to Russia. And your humble servant hasn’t been elected by the people of Russia to be pleasant to someone either,” Putin said of their relationship.

Putin said it was “ludicrous” that the government of 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 realising quite well that it could serve as a pretext for applying sanctions against them, including the use of force,” the Russian President said.

“If there are data that the chemical weapons have been used, and used specifically by the regular army, this evidence should be submitted to the UN Security Council,” added Putin, a former officer in the Soviet’s committee for state security. “And it ought to be convincing. It shouldn’t be based on some rumors and information obtained by special services through some kind of eavesdropping, some conversations and things like that.”

Putin noted that even in the US, “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.”

Putin 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 US called a mistake. Did we forget about that?” Putin said.

Putin said he “doesn’t exclude” backing the use of force against Syria at the UN 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 UN approval, saying it would represent an aggression.

Putin reinforced his demand that before taking action, Obama needed approval from the UN Security Council. Russia can veto resolutions in the 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 was “too early” to talk about what Russia would do if the US attacked Syria.

“We have our ideas about what we will do and how we will do it in case the situation develops toward the use of force or otherwise,” Putin said. “We have our plans.”

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," Putin added.

“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,” he said.

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 US and Israeli pressure.

Putin also accused US intelligence agencies of bungling efforts to apprehend Snowden, the National Security Agency leader, who is wanted in the US on espionage charges. He 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 only learned that Snowden was on the flight two hours before it arrived.