U.S. President Barack Obama arrived on Wednesday in Saint Petersburg for the G20 summit in Russia, where officials said he would argue his case for military action against the Syrian regime over an alleged chemical weapons attack.

With a serious expression, Obama jogged down the steps off his Air Force One plane which touched down at Saint Petersburg's Pulkovo airport ahead of the opening by Russian President Vladimir Putin of the summit later in the afternoon.

The U.N.-Arab League envoy on Syria Lakhdar Brahimi will attend the G20 summit in Russia to help Secretary General Ban Ki-moon's efforts to hasten a peace conference aimed at ending the conflict, the U.N. said Thursday.

"The Secretary General has just announced that the Joint Special Representative for Syria, Lakhdar Brahimi, is on his way to Russia to help him push, on margins of the G20 summit in Saint Petersburg, for the International Conference on Syria," the U.N. spokesperson said in a statement.

Russia said Thursday it would warn a meeting of the U.N.'s atomic watchdog next week that any U.S. military strikes in Syria could hit a nuclear research reactor there.

"Russia will for sure raise this topic at the autumn session of the board of governors of International Atomic Energy Agency (IAEA) opening on September 9 in Vienna," a foreign ministry spokesman told Agence France Presse.

Russian naval vessels in the Mediterranean are capable of reacting to an escalation in the Syria conflict, a military source said Wednesday, as Moscow fine tunes its maritime presence ahead of possible U.S. military action.

"Today we consider our presence in the eastern Mediterranean to be sufficient to solve the tasks. If necessary, together with submarine forces, they (the ships) are capable even today of influencing a military situation," a general staff source told the Interfax news agency.

U.S. President Barack Obama must seize the opportunity offered by the G20 summit to settle bitter differences over Syria with Russian counterpart Vladimir Putin, former Soviet leader Mikhail Gorbachev said Wednesday.

The G20 summit, which starts in Russia on Thursday, comes with many analysts describing U.S.-Russia relations as at their lowest ebb since Gorbachev helped end the Cold War two decades ago.

Russian President Vladimir Putin on Wednesday warned the U.S. Congress it would be approving an "aggression" against Syria if it allowed U.S.-led military strikes against the regime of President Bashar Assad.

"They would be allowing an aggression since everything that is outside the framework of the U.N. Security Council is an aggression, unless it is self-defense," Putin told members of his human rights council in the Kremlin, Russian news agencies reported.

Russian President Vladimir Putin said Wednesday U.S. intelligence leaker Edward Snowden, who received asylum in Russia, is a "strange guy" who condemned himself to a difficult fate.

"You know, I sometimes thought about him, he is a strange guy," ex-KGB spy Putin said in an interview with state-run Channel One television.

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 with The Associated Press and Russia's state Channel 1 television, 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.

U.S. President Barack Obama arrived in Sweden Wednesday for a two-day visit likely to revolve around Syria, despite the host nation's effort to draw attention to a broader agenda.

Obama stepped off Air Force One at Stockholm's Arlanda Airport fresh from efforts in Washington to secure bipartisan support for military strikes against Syria to punish the regime for what the U.S. says was the use of sarin gas on a Damascus suburb.

Israel on Tuesday launched a missile over the Mediterranean Sea in a joint exercise with the United States which came as Washington mulls long-range strikes against Syria.

Speculation mounted after the missile firing at 0615 GMT, with Russia's defense ministry, quoted by news agencies, saying its early warning system had detected the launch of two ballistic missiles from the central Mediterranean fired towards the sea's eastern coastline, on which Syria lies.