By William Maclean and Oliver Holmes
BEIRUT, Aug 28 (Reuters) - U.N. chemical weapons investigators crossed Syria's front line into rebel-held territory on Wednesday for a second visit to the scene of a poison gas attack that has triggered Western plans for war.
U.S. President Barack Obama and his European and Middle East allies have already blamed President Bashar al-Assad for last week's killing of hundreds of civilians. But the U.N. experts, who were first allowed to cross the front line by Assad's forces on Monday, are still engaged in gathering evidence.
As long as inspectors remain in Syria, Western attacks are improbable, given the risk they would pose to the team's safety. A Reuters journalist saw U.N. cars leave a hotel in government-controlled central Damascus. An opposition activist later said they were starting work in the eastern suburb of Zamalka.
U.N. Secretary General Ban Ki-moon stressed that the inspection team must be given "time to do its job".
With the price of oil and gold soaring and world stock markets hit by fears of an unpredictable new phase of the Syrian civil war, which has split the Middle East on sectarian lines, only the timing of air strikes still seems in much doubt.
The aim, governments say, is to punish Assad's use of banned weapons but not, the White House insists, to impose "regime change" and end a conflict now in its third year. But Russia and China, set on blocking U.N. backing, warn that what they see as an illegal assault on a sovereign state could inflame the war.
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.
No Western attack, expected to involve cruise missiles fired by U.S. ships in the Mediterranean, is likely before Obama has a U.S. intelligence report on the August 21 gas attack on rebel-held suburbs of the capital. Its conclusions, however, are scarcely in doubt. Numerous officials have already blamed Assad.
"There is no doubt who is responsible for this heinous use of chemical weapons in Syria," U.S. Vice President Joe Biden said on Tuesday. "The Syrian regime."
A spokesman for British Prime Minister David Cameron said he and Obama spoke on Tuesday and had "no doubt" of Assad's guilt.
Air strikes are unlikely before Cameron has given 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 win a vote but with voters wary of new military entanglements after over a decade of war in Iraq and Afghanistan, Cameron, like fellow Western leaders, has no wish to be accused of defying public opinion.
Many British lawmakers across the political spectrum uneasy about the prospect of air strikes. A YouGov poll published on Wednesday showed 50 percent of the British public opposed a missile strike on Syria, with just 25 percent in favor.
Rallying international opinion is also a concern.
Russia, Assad's main arms supplier, has made clear it will not back any U.N. Security Council resolution of the kind which has given international legal cover to some previous wars - including the NATO bombing of Libya two years ago.
Russian Foreign Minister Sergei Lavrov told U.N. Syria envoy Lakhdar Brahimi that attacking Syria would destabilize the country and the region, the Foreign Ministry said in Moscow.
Syria's war has heightened tensions between Assad's sponsor Iran and Israel, which bombed Syria this year; it has killed over 100,000 and driven millions across borders into Turkey, Lebanon, Jordan and Iraq; and it has fuelled sectarian bloodshed in Lebanon and Iraq, where bombs killed 44 on Wednesday alone.
Iranian Supreme Leader Ayatollah Ali Khamenei said on Wednesday that U.S. action would be "a disaster for the region".
China, too, is wary of what it sees as Western interference in the affairs of others. 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 a decade ago, Washington and its allies wanted Assad out.
However, 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 U.N. 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 Francois 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 Russian and Chinese opposition at the United Nations would not prevent military action. "We cannot allow diplomatic paralysis to be a shield for the perpetrators of these crimes," he wrote.
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."
Opposition activists have said at least 500 people, and possibly twice that many, were killed last Wednesday before dawn by rockets carrying the nerve gas sarin or something similar.
If true, it would be the worst chemical weapons attack since Saddam Hussein gassed thousands of Iraqi Kurds in 1988.
(Additional reporting by Erika Solomon and Mariam Karouny in Beirut, Steve Gutterman in Moscow and Ben Blanchard in Beijing; Writing by Alastair Macdonald; Editing by Mark Heinrich)