By Mark Hosenball and Oliver Holmes
WASHINGTON/BEIRUT, Aug 29 (Reuters) - President Barack Obama and his allies sought to convince cautious lawmakers and the public of the need to strike Syria although officials conceded on Thursday they lacked conclusive evidence that President Bashar al-Assad ordered his forces to use chemical weapons against civilians.
Obama told Americans on Wednesday evening that a military strike against Syria was in their interest following the gas attack and Britain said armed action would be legal, but intervention looked set to be delayed until U.N. investigators report back after leaving Syria on Saturday.
The U.S. and its allies have "no smoking gun" proving Assad personally ordered the attack on a rebel-held Damascus neighbourhood in which hundreds of people were killed, U.S. national security officials said.
In secret intelligence assessments and a still-unreleased report summarizing U.S. intelligence on the alleged gas attack on Aug. 21, U.S. agencies expressed high confidence that Syrian government forces carried out the attack, and that Assad's government therefore bears responsibility, U.S. national security officials said.
But the United States and its allies have "no smoking gun" proving Assad personally ordered the attack on a rebel-held Damascus neighbourhood, the officials said
Senior Obama administration officials are expected to brief congressional leaders later on Thursday, with some lawmakers complaining they have not been properly consulted.
While U.N. chemical weapons inspectors spent a third day combing the rebel-held area where the attack took place, elsewhere in Damascus traffic moved normally, with some extra army presence but little indication of any high alert.
A parliamentary debate in London revealed deep misgivings stemming from the 2003 invasion of Iraq. After pressure from lawmakers, the British government - a key player in any proposed air assault on Syria - has promised parliament a decisive vote once the U.N. weapons inspectors report their findings.
The United Nations said its team of inspectors investigating the attacks, which killed hundreds of people, will leave Syria on Saturday and then report to Secretary-General Ban Ki-moon.
France and Germany urged the world body to pass its report on to the decision-making Security Council as soon as possible "so that it can fulfil its responsibility with regards to this monstrous crime".
The United States, Britain and France say they can act with or without a U.N. Security Council resolution, which is likely to be vetoed by Russia, a close ally of Syrian President Bashar al-Assad. However, some countries are more cautious: Italy said it would not join any military operation without Security Council authorisation.
Western diplomats say they are seeking a vote in the 15-member Council to isolate Moscow and demonstrate that other countries are behind air strikes.
The five permanent U.N. Security Council members will meet again on Thursday afternoon, U.N. diplomats said, after an inconclusive meeting on Wednesday on a draft Security Council resolution that would authorize "all necessary force" in response to the alleged gas attack.
"It would be unthinkable to proceed if there was overwhelming opposition in the (U.N.) Security Council," British Prime Minister David Cameron told parliament.
He published legal advice given to the government, under which military action would be lawful for humanitarian reasons even if a Security Council resolution were blocked.
The International Committee of the Red Cross joined a chorus of international voices urging caution.
"Further escalation will likely trigger more displacement and add to humanitarian needs, which are already immense," said Magne Barth, head of the ICRC delegation in Syria.
Increasing expectations that any action will be delayed ended a three-day sell off on world share markets on Thursday, although investors were still on edge over fears of future turmoil in the Middle East.
"SHOT ACROSS THE BOW"
Obama sought to win over a war-weary American public on Wednesday evening by saying intervention in Syria, where more than 100,000 people have been killed in two and a half years of civil war, would serve U.S. national security interests.
"If we are saying in a clear and decisive but very limited way, we send a shot across the bow saying, 'Stop doing this,' this can have a positive impact on our national security over the long term," he told "PBS Newshour" in a televised interview.
While saying he had not yet made a decision on military action, Obama left little doubt the choice was not whether but when to punish Syria for the gas attacks.
"We have concluded that the Syrian government in fact carried these out," he said on Wednesday evening.
Syria denies blame for the gas attacks and says they were perpetrated by rebels. Washington and its allies say the denial is not credible.
According to the U.S. national security officials, evidence that forces loyal to Assad were responsible goes beyond the circumstantial to include electronic intercepts and some tentative scientific samples from the site.
"This was not a rogue operation," one U.S. official said.
However the evidence does not prove that Assad himself ordered that chemical munitions be used, according to the officials, who spoke on condition of anonymity.
A report from Moscow that Russia plans to send two warships to the eastern Mediterranean underscored the complications surrounding even a limited military strike, although Russia has said it will not be drawn into military conflict.
G20 TO MEET IN RUSSIA
Western leaders are expected in Russia next Thursday for a meeting of the Group of 20 big economies, an event that could influence the timing of any strikes. The hosts have made clear their view that Western leaders are using human rights as a pretext to impose their will on other sovereign states.
"At this stage it is necessary to take all needed actions to avert possible negative developments ... or some kind of military action regarding Syria," Russian Deputy Foreign Minister Gennady Gatilov told state-run Rossiya-24 television. "And that is what we ... focusing our efforts on now."
A spokesman for the main Syrian opposition umbrella group, the Syrian National Coalition, said the opposition was confident Western leaders were prepared to act.
SNC leader Ahmed Jarba met French President Francois Hollande. An SNC spokesman said they discussed a two-wave intervention to first target installations used to launch chemical weapons and then hit other government bases in Syria.
"We are very happy. France and its partners are quite decided to punish the Syrian regime," SNC envoy Monzer Makhous told Reuters after the talks. "Then there will be military aid to help the opposition to change the balance of power."
Hollande urged Jarba to create a credible military force, highlighting Western concern that the mainstream opposition is unable to control al Qaeda-linked militias on the ground in Syria. Syrian officials say the West is playing into the hands of its al Qaeda enemies.
In Damascus, residents and opposition forces said Assad's forces appeared to have evacuated most personnel from army and security command headquarters in the centre in preparation for Western military action.
People unable to decide whether to leave for neighbouring Lebanon said the border was already jammed.
"We're hearing people are spending hours - like 12 or 14 hours - waiting in line at the border," said Nabil, who was considering leaving town for Beirut with his wife and young daughter, "just until the strike is over."
Arguing for measured intervention after long resisting deeper involvement in Syria, Obama insisted that while Assad's government must be punished, he intended to avoid repeating U.S. errors from the Iraq war.
"I have no interest in any open-ended conflict in Syria, but we do have to make sure that when countries break international norms on weapons like chemical weapons that could threaten us, that they are held accountable," Obama said.
Despite opinion polls showing most Americans oppose deeper involvement in the Syrian conflict, Obama has been under pressure to enforce a "red line" against chemical weapons use, which he declared just over a year ago.
The likeliest option, U.S. officials say, would be to launch cruise missiles from U.S. ships in the Mediterranean in a campaign that would last days.
Obama cited chemical weapons dangers to U.S. Middle Eastern allies Israel, Turkey and Jordan and U.S. bases in the region, and said America's national interests could be at risk if Syrian chemical arms fell into the wrong hands. (Additional reporting by Matt Spetlanick, Patricia Zengerle and Jeff Mason in Washington, Erika Solomon in Beirut, Khaled Yacoub Oweis in Amman, Catherine Bremer in Paris, Sarah Marsh in Berlin, Timothy Heritage in Moscow and Andrew Osborn and Peter Apps in London; Writing by Philippa Fletcher and Claudia Parsons; Editing by Peter Graff and David Storey)