* Obama says U.S. planning limited military action

* Syria accuses U.S. of "lies" to justify an attack

* U.N. chief sees final results on poison gas tests in two weeks

* Rebels hope to capitalise on any U.S. military strikes

By Erika Solomon

BEIRUT, Aug 31 (Reuters) - U.N. experts investigating a poison gas attack in Syria left on Saturday, paving the way for the United States to lead military strikes to punish Syria's President Bashar al-Assad.

U.S. President Barack Obama said the United States, which has five cruise-missile equipped destroyers in the region, is planning a "limited, narrow" military action to punish Assad for an attack that Washington said killed 1,429 people.

"We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale," Obama said on Friday after Washington unveiled an intelligence assessment concluding Assad's forces were to blame for the attack.

After laying out the case in a televised speech, U.S. Secretary of State John Kerry spoke on Friday to the foreign ministers of European and Gulf allies, as well as the head of the Arab League, a senior State Department official said.

The team of U.N. experts drove up to Beirut International Airport on Saturday after crossing the land border into Lebanon by road earlier in the day. No Western intervention had been expected as long as they were still on the ground in Syria.

The 20-member team, which had arrived in Damascus three days before the Aug. 21 attack to investigate earlier accusations, eventually visited the sites several times, taking blood and tissue samples from victims in rebel-held suburbs of Damascus.

Inspectors also took samples of soil, clothing and rocket fragments. However, their mandate is to determine whether chemicals were used, not who used them.

Washington says it does not need to wait for the inspectors to report, since it is already certain chemical weapons were used and convinced that Assad's forces were behind the attack.

U.S. forces are likely to launch strikes jointly with France, which has strongly backed the use of force to punish Assad.

"The chemical massacre in Damascus cannot and must not go unpunished. Otherwise we'd run the risk of an escalation that would trivialise the use of these arms and put other countries at risk," French President Francois Hollande told Friday's Le Monde newspaper in an interview.

"REBEL PROVOCATION"

Britain also strongly backed action, but was forced to pull out of the coalition after Prime Minister David Cameron unexpectedly lost a vote over it in parliament on Thursday.

Turkey backs the use of force and Arab states in the region say Assad should be punished, although they have mainly stopped short of explicitly endorsing military strikes against him. Iran, which supports Assad, has warned of wider war.

Kerry said Washington must act to protect itself and its allies, including Syria's neighbours Turkey, Jordan and Israel, from future use of banned weapons.

"If we choose to live in the world where a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity" it would embolden others, such as Iran, Hezbollah and North Korea, Kerry said.

Syria and its main ally Russia say rebels carried out the attack as a provocation. Moscow has repeatedly used its U.N. Security Council veto to block forceful action against the Syrian leader and says any attack on Syria would be illegal and only inflame the civil war there.

"I am convinced that it (the chemical attack) is nothing more than a provocation by those who want to drag other countries into the Syrian conflict, and who want to win the support of powerful members of the international arena, especially the United States," President Vladimir Putin said.

Syria's Foreign Ministry repeated its denial that the government had used chemical weapons against its own people. Kerry's accusations were a "desperate attempt" to justify a military strike. "What he said was lies," the ministry said.

Washington says the Syrian denials are not credible, and the rebels would not have been able to launch such an attack.

The White House was to brief Republican senators on Syria in a conference call on Saturday at the request of Senate Minority Leader Mitch McConnell, a spokesman for the senator said.

RESIDENTS PREPARE FOR STRIKES

In Syria itself, residents in and around Damascus readied themselves for a strike.

A man named Youssef carried a small plastic bag bulging with documents. "Do I put them in my parents' home? My in-laws? At work? I don't know which area is safer, I don't know where to hide them," he told a friend.

"That's my marriage certificate, my passport, my home ownership deed, my college degree, and all my wife's documents too. We can't figure out where to put them for safekeeping."

Doctors in the outskirts of the capital said they were training up teams and trying to secure shipments sent in by aid groups of atropine and oxygen, which are needed for treating poison gas victims.

"We worry about another chemical weapons attack should foreign powers carry out the strike, as some kind of revenge, or who knows what could happen," said a doctor in the rebel-held suburb of Arbin, called Abu Akram.

Rebels said they were planning to take advantage of a strike to launch an offensive. Qassim Saadeddine, a former Syrian army colonel and spokesman for the rebels' Supreme Military Council, said rebel groups had been sent a military plan of action.

"The hope is to take advantage when some areas are weakened by any strikes. We ordered some groups to prepare in each province, to ready their fighters for when the strike happens," he told Reuters, speaking by Skype.

"They were sent a military plan that includes preparations to attack some of the targets we expect to be hit in foreign strikes, and some others that we hope to attack at the same time." (Writing by Elizabeth Piper and Peter Graff; Editing by Jon Boyle)

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