China has joined Russia in opposing military strikes on Syria, saying it would push up oil prices and create an economic downturn.

The Chinese intervention came as G20 leaders gathered in Saint Petersburg on Thursday for a summit likely to be dominated by Syria. The Russian president, Vladimir Putin, is expected to allow the issue on to the agenda for dinner, reflecting the reality that the fate of the world economy is inextricably intertwined with the risk of a Middle East conflagration.

The Chinese deputy finance minister, Zhu Guangyao, told a pre-G20 briefing: "Military action would have a negative impact on the global economy, especially on oil prices – it will cause a hike in the oil price."

The UN special envoy on Syria, Lakhdar Brahimi, is rushing to the summit to update G20 leaders on the state of stalled peace talks.

It also emerged that the pope had written to Putin, reportedly saying military action would be a futile pursuit.

In a sign of tensions before the summit, Putin called the US secretary of state a liar for claiming al-Qaida did not have a significant presence in Syria. He said US bombing of Syria could lead to highly dangerous attacks on Syrian nuclear reactors.

EU leaders have expressed concern at the pace of the drive to war. The president of the European commission, José Manuel Barroso, said consensus in the international community was needed on Syria and argued that efforts should be focused on a political solution.

Herman Van Rompuy, president of the European council, said: "No military solution to the Syrian conflict."

Italy's prime minister, Enrico Letta, said the summit was the last opportunity for negotiated and political solutions to Syria's civil war. He urged Putin to avoid a final break with Washington over Syria, adding that concern over Syria had hit maximum levels.

In contrast, the French foreign minister said: "The position of France is to punish and negotiate." Laurent Fabius told France 2 television before travelling to the summit: "We are convinced that if there is no punishment for Mr Assad, there will be no negotiation. Punishment will allow negotiation, but obviously it will be difficult."

The French parliament has discussed President François Hollande's decision to join the planned US air strikes, but did not hold a vote.

The UN secretary general, Ban Ki-moon, referring to Brahimi's travel to Saint Petersburg, said: "While the world is focused on concerns about the possible use of chemical weapons in Syria we must push even harder for the international conference on Syria to take place in Geneva. A political solution is the only way to end the bloodshed in Syria."

Western leaders are likely to pile pressure on Putin by challenging his claims that chemical weapons have not been used by the Assad regime. German intelligence was the latest to release new information linking the chemical attacks on 21 August to Assad forces. Syria rejects the charge and, like Russia, blames the rebels.

David Cameron flew in to Saint Petersburg from the UK, and Barack Obama arrived from Sweden, which he had visited on Wednesday.

Cameron is not expected to hold a formal bilateral meeting with the US president, who is leading the international drive for armed reprisals for Assad's apparent chemical weapons attacks.

The White House says Obama will also not hold formal one-on-one talks with Putin, who is hosting the summit.

Putin is threatening to send a missile shield to Syria if the US launches an attack without the authority of the United Nations.

The G20 summit had been expected to focus on the world economy and growth, but will now be dominated by the Middle East crisis, even if the formal agenda remains fixed on the slowdown of growth in emerging markets.

Obama, speaking in Sweden before the summit, denied that his political credibility was at stake but admitted relations with Russia had hit a wall. He said he had not set the red lines requiring a military response if the Syrian government deployed chemical weapons.

"The world set a red line when governments representing 98% of the world's population said the use of chemical weapons was abhorrent and passed a treaty forbidding their use even when countries are engaged in war," he said. "That was not something I just kind of made up, I did not pluck it out of thin air."

He added: "My credibility is not on the line. The international community's credibility is on the line because we give lip service to the notion that these international norms are important.

"Keep in mind, I'm somebody who opposed the war in Iraq, and I'm not interested in repeating mistakes about basing decisions on faulty intelligence," the US president said at a news conference in Stockholm.

On Tuesday Obama had portrayed his plans for US military action as part of a broader strategy to topple Assad, as the White House's campaign to win over sceptics in Congress gained momentum.

Obama arrives at the summit with his hand strengthened by the growing impression that he will win the support of Congress next week to take military action. In signs that the political tide was slowly turning his way in Washington, the Senate foreign relations committee on Wednesday agreed on a draft resolution backing the use of US military force in Syria.

It authorises strikes against the Syrian regime within a 60-day window, extendable to 90 days, as requested by the White House. But it also includes tougher wording introduced by the hawkish Republican senator John McCain, which makes it "the policy of the United States to change the momentum on the battlefield in Syria".

Obama also retains the support of the French and the personal backing of Cameron, even though the British government is now debarred from joining any action owing to last week's mishandled Commons vote.

The resolution will be put before the full Senate for a vote on Monday, where it is expected to pass. Obama faces a tougher battle in the House of Representatives, whose foreign affairs committee heard testimony from the secretary of state, John Kerry, on Wednesday. Kerry warned a sceptical and sometimes raucous panel that failing to strike Syria would embolden al-Qaida and raise to 100% the chances that Assad would use chemical weapons again.

Cameron is expected to announce further British intelligence to persuade Putin that Assad forces were responsible. He is also likely to press on the need for clear humanitarian corridors in Syria to boost aid to trapped refugees, as well as call for an end to the bureaucratic delays preventing aid workers reaching Syria.