China came out firmly in support of Russian opposition to US air strikes in Syria, warning that military intervention would cause a dangerous hike in oil prices.

As ministers from the G20 most powerful economies met in St Petersburg, China's vice finance minister Zhu Guangyao said any military intervention would hurt the world economy.

"Military action would have a negative impact on the global economy, especially on the oil price. It will cause a hike in the oil price," Zhu told a briefing before the talks got under way.

In Beijing, foreign ministry spokesman Hong Lei called for those responsible for launching chemical weapons attacks to admit responsibility.

However, he said unilateral military action by the US would violate international law and aggravate Syria's civil war.

The warnings came after Russian President Vladimir Putin ratcheted up tensions with the US, accusing Secretary of State John Kerry of lying in his testimony to Congess aimed at securing backing for the strikes.

Putin claimed Kerry had misled the hearing by playing down the role of al-Qaida forces fighting the regime of Bashar al-Assad.

"As a matter of fact, the principal combative unit is the so-called Nusra, which is an al-Qaeda unit," said Putin. "And they know this, and he is telling an outright lie, and he knows that he is lying."

Putin, who will host the summit, blamed the US for "sanctioning aggression", adding: "Anything that is beyond the UN Security Council framework except self-defence is aggression."

His remarks followed a press conference by President Obama in which he said the world "had to act" over the Assad regime's use of chemical weapons, which the White House says killed 1,400 pople in suburbs of Damascus on 21 August.

Speaking in Stockholm on Wednesday, Obama said the world would increasingly face "situations like Syria, Kosovo, Rwanda".

"We as a country and the world are going to have to take tough choices," said Obama. "We may not always have a Security Council that can act".

Russia and China have made clear they will veto action on Syria in the Security Council.