A Free Syrian Army fighter gestures to his fellow fighters as they take cover behind a wall near Aleppo International airport August 28, 2013.[Photo/Agencies]

INVESTORS, RESIDENTS ALARMED

With only the timing of an attack apparently in doubt, oil prices soared to a six-month high, lifting US energy shares and the overall US market.

But some emerging markets closed lower again on Wednesday because of investor jitters over where the international escalation of Syria's civil war might lead - however much Obama and his allies may hope to limit it to a short punitive mission.

Neighbouring Turkey, a NATO member, put its forces on alert. Israel mobilised some army reservists and bolstered its defences against missile strikes from either Syria or Lebanon.

Syria's envoy to the United Nations said he had asked Ban to have the team investigate three new attacks by rebel groups.

People in Damascus, wearied by a civil war that has left the capital ringed by rebel-held suburbs, braced for air strikes.

In a city where dozens of military sites are mixed in among civilian neighbourhoods, some were leaving home in the hope of finding somewhere safer, although many doubted it was worth it.

"Every street, every neighbourhood has some government target," said a nurse in the city centre. "Where do we hide?"

At grocery stores, shoppers loaded up on bread, dry goods and cans. Bottled water and batteries were also in demand.

TIMING CALCULATIONS

Numerous factors, including weather and assessments of Syrian air defences, may affect the timing of strikes. Analysts expect cruise missiles to be launched from US ships in the Mediterranean. Aircraft could also play a role, as may forces from other NATO powers, notably Britain and France.

Obama is waiting for a US intelligence report, although its findings are in little doubt. US officials have already blamed Assad for the attacks on August 21. US sources suggested that the intelligence cache included intercepted communications between Syrian officials but that those contained no "smoking gun" and were not likely to be declassified for public release.

British Prime Minister David Cameron has recalled parliament to debate the Syria crisis on Thursday. He should be able to secure cautious support, despite widespread misgivings among Western voters about new entanglements in the Muslim world. But British action is unlikely before lawmakers have had their say.

Although decisive action against Syria is strongly backed by many in the US Congress, a growing number of lawmakers are pressing the president to consult them and receive congressional authorization before ordering use of force.

US House of Representatives Speaker John Boehner urged Obama on Wednesday to make the case personally to Congress and the American people for potential military action in Syria.

The prospect of a Group of 20 summit in St. Petersburg next Thursday may also weigh in calculations over timing any strikes. Russian host President Vladimir Putin has made clear his view that Western leaders are using human rights as a pretext to impose their will on other sovereign states.

"The West behaves like a monkey with a grenade in the Islamic world," Russian Deputy Prime Minister Dmitry Rogozin tweeted on Wednesday. Western leaders in the G20 may prefer to have any strikes on Syria completed before the summit starts.

As diplomats from Russia, China, Britain, France and the United States met at the United Nations, Moscow said Britain was "premature" in seeking a Security Council resolution for "necessary measures" to protect Syrian civilians.

But US State Department spokeswoman Marie Harf told reporters, "The Syrians cannot continue to hide behind Russian intransigence at the Security Council."

British Foreign Secretary William Hague said Russia and China might veto the move but added, "It's time the UN Security Council shouldered its responsibilities on Syria which for the last two and a half years it has failed to do."

A senior Western diplomat said, "Of course there will be a Russian veto, but that's part of the objective - to show that we tried everything and the Russians left us no choice.

"The Americans want to go quickly."