The United Nations Security Council was set for a showdown over Syria on Wednesday after the United Kingdom sought authorization for Western military action that seems certain to be vetoed by Russia.

UN chemical weapons experts investigating a purported gas attack that killed hundreds of civilians in rebel-held suburbs of Damascus made a second trip across the front line to take samples.

But the United States and European and Middle East allies have already pinned the blame on the Syrian government and, even without full UN authorization, US-led air or missile strikes on Syria look all but certain, though the timing is far from clear.

Meanwhile, evidence suggests that some kind of chemical "substance" was used in Syria that may have killed more than 1,000 people, but any military strike in response must first gain UN Security Council approval, UN Special Envoy to Syria Lakhdar Brahimi said on Wednesday.

"With what has happened on the 21st of August last week, it does seem that some kind of substance was used that killed a lot of people: hundreds, definitely more than a hundred, some people say 300, some people say 600, maybe 1,000, maybe more than 1,000 people," Brahimi said.

"This was of course unacceptable. This is outrageous. This confirms how dangerous the situation in Syria is and how important for the Syrians and the international community to really develop the political will to address this issue seriously, and look for a solution for it," he said.

The development has set Western leaders on a collision course with Moscow, which has a veto in the Security Council and disapproves of what it sees as a push for Iraq-style "regime change" - despite US denials that President Barack Obama aims to overthrow Syrian President Bashar al-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 and gold to their highest levels in months, while stocks fell. Fears over the economy of Syria's hostile neighbor Turkey pushed its lira to a record low.

Germany, Europe's economic superpower, urged Russia to back the resolution that the UK will propose for the United Nations Security Council. But Russian Foreign Minister Sergei Lavrov had already said earlier in the day that any attack would be folly.

Brahimi said "international law is clear" in requiring Security Council authorization for any military action. But Western leaders have made clear they are ready to proceed without it, citing precedents for foreign intervention to protect civilians.

Reuters-AP