The United Nations Security Council was set for a showdown over Syria on Wednesday after Britain sought authorization for Western military action that Russia called premature.
U.N. chemical weapons experts investigating an apparent gas attack that killed hundreds of civilians in rebel-held suburbs of Damascus made a second trip across the front line to take samples. Secretary-General Ban Ki-moon pleaded for them to be given the time they need to complete their mission.
But the United States and European and Middle East allies have already pinned the blame on President Bashar al-Assad's forces. Even if Russia blacks U.N. approval, U.S.-led air or missile strikes on Syria look all but certain, though the timing is far from clear.
That has set Western leaders on a collision course with Moscow, Assad's main arms supplier, as well as with China, which also has a veto in the Security Council and disapproves of what it sees as a push for Iraq-style "regime change" - despite U.S. denials that President Barack Obama aims to overthrow Assad.
Uncertainty over how the escalation of the conflict at the heart of the oil-exporting Middle East will affect trade and the world economy sent oil prices to their highest levels in six months, while stocks fell. Fears over the economy of Syria's hostile neighbor Turkey pushed its lira to a record low.
Prime Minister David Cameron said Britain would propose a resolution on Wednesday at the Security Council in New York, seeking authority to take "necessary measures" to protect Syrian civilians. Sure of a veto, it seemed part of diplomatic strategy to isolate Moscow and rally a broad coalition behind Washington.
"We've always said we want the U.N. Security Council to live up to its responsibilities on Syria. Today they have an opportunity to do that," Cameron said in a statement.
Foreign Minister Sergei Lavrov said earlier in the day that any attack would be folly. One of his deputies responded to Cameron by saying the Council should wait for the U.N. inspectors' report.
"It would be premature, at the least, to discuss any Security Council reaction until the U.N. inspectors working in Syria present their report," Vladimir Titov said.
Ban pleaded for unity in the Security Council after more than two years of paralysis during which Syria's civil war has split the Middle East on sectarian lines and fuelled rival camps in the world body along divisions that echo the Cold War.
"Syria is the biggest challenge of war and peace in the world today," Ban said in a speech at The Hague. "The body entrusted with maintaining international peace and security cannot be missing in action. The Council must at last find the unity to act. It must use its authority for peace."
Ban's special envoy for Syria, Algerian diplomat Lakhdar Brahimi, said "international law is clear" in requiring Council authorization for any military action. But Western leaders have made clear they are ready to do without it, citing precedents for foreign intervention to protect civilians.
Rebel fighters and opposition activists said they showed U.N. inspectors homes in the eastern Damascus suburb of Zamalka that had been hit by last week's gas release. The experts would also be testing and interviewing survivors, as they did on a first trip on Monday that came under sniper attack.
Amateur video showed the convoy of white U.N. jeeps driving along a road, accompanied by rebels. One pick-up truck was mounted with an anti-aircraft gun. Gunmen leaned from the windows of another. Bystanders waved as the vehicles passed.
As long as the U.N. team is in Syria, Western action is less likely - making the presence of the investigators led by Swedish scientist Ake Sellstrom a key element in the timing of attacks, expected to be limited to a few days.
Strikes, expected to involve cruise missiles fired by U.S. ships in the Mediterranean, are also unlikely before Obama has an intelligence report on the August 21 gas attack. Its conclusions, however, are scarcely in doubt.
Cameron will give the British parliament an opportunity to be seen to support his policy in a debate scheduled for Thursday. Like the United States, Britain has warships in the Mediterranean. It also has an air base on Cyprus, 200 km (120 miles) from the Syrian coast.
The British government is not obliged to hold a vote, but with voters wary of new military entanglements after more than a decade of war in Iraq and Afghanistan, Cameron will want to show that he has broad backing. A YouGov poll published on Wednesday showed 50 percent of the British public opposed a missile strike, with just 25 percent in favor.
The French parliament was also recalled on Wednesday, but only for a session set for next Wednesday, September 4.
Syria's war has killed more than 100,000 people and driven millions from their homes, many crossing borders into Turkey, Lebanon, Jordan and Iraq.
It has heightened tensions between Assad's sponsor Iran and Israel, which bombed Syria this year, and has fuelled sectarian bloodshed in Lebanon and in Iraq, where bombs killed more than 70 people on Wednesday alone.
Iranian Supreme Leader Ayatollah Ali Khamenei said on Wednesday that U.S. action would be "a disaster for the region".
Like Russia, China is wary of Western interference in the affairs of sovereign states. The official People's Daily newspaper said air strikes would add "oil to the flames of Syria's civil war" and added that, as in the overthrow of Saddam Hussein in Iraq, Washington and its allies wanted Assad out.
Western governments have rallied support from the Arab League and Syria's Muslim neighbor and NATO member Turkey and have begun to lay out arguments they say show that they can satisfy some criteria of international law.
Australia, which takes over the chair of the Security Council on Sunday, added its voice on Wednesday to the Western view that continuing deadlock along Cold War lines in the top United Nations body would not rule out an attack on Syria.
"Everyone's preference would be for action, a response, under United Nations auspices," Foreign Minister Bob Carr said.
"But if that's not possible, the sheer horror of a government using chemical weapons against its people, using chemical weapons in any circumstances, mandates a response."
French President Hollande has cited a 2005 U.N. provision for action to protect civilians from their own governments, which was inspired by the Rwandan genocide of 1994.
Similar arguments were used by NATO to bomb Russian ally Serbia in 1999 after the killing of civilians in Kosovo.
On Wednesday, as Britain's National Security Council prepared to meet on Syria, Foreign Secretary William Hague sought to justify an attack on the grounds of defending Britain's own interests.
"We cannot permit our own security to be undermined by the creeping normalization of the use of weapons that the world has spent decades trying to control and eradicate," he wrote in Britain's Daily Telegraph newspaper.
Hague warned that Syria's chemical weapons could "fall into the wrong hands". It was unclear how a limited campaign of air strikes would address that - analysts say chemical weapons dumps are unlikely to be bombed for fear of spreading the toxins.
Critics also argue that defeating Assad may carry greater risks of handing his arsenal to the likes of al Qaeda.
Hague further said opposition at the United Nations would not prevent action. "We cannot allow diplomatic paralysis to be a shield for the perpetrators of these crimes," he wrote.
However, having demanded that Assad end his family's four-decade rule since Syrians rose up against him during the Arab Spring of 2011, Western powers have hesitated to arm the rebels, fearing the rise of Islamist militant groups in their ranks.
In a mark of the complexities of the region, Assad faces not just retribution from neighboring countries and the West but from al Qaeda. The Islamic State in Iraq and the Levant (ISIL) said it was launching an operation called "Volcano of Revenge" that would strike Syrian security forces in Damascus.
Syria's government denies gassing its own people and has vowed to defend itself, but people in Damascus are anxious.
"I've always been a supporter of foreign intervention, but now that it seems like a reality, I've been worrying that my family could be hurt or killed," said a woman named Zaina, who opposes Assad. "I'm afraid of a military strike now."