WRAPUP 7-U.S. repositions naval forces, no decision on Syria strike
* U.S. officials to meet to discuss response
* Obama calls apparent gas attack "of grave concern"
* Says ability of U.S. to solve crisis "overstated"
* Activists say tissue samples being sent to U.N. team
(Recasts with naval forces repositioning, updates throughout, previous Beirut)
WASHINGTON, Aug 23 (Reuters) - The United States on Friday was repositioning naval forces in the Mediterranean to give President Barack Obama the option for an armed strike on Syria, although officials cautioned that Obama had made no decision on military action.
A defense official, speaking on condition of anonymity, said the U.S. Navy would expand its presence in the Mediterranean to four destroyers from three.
Secretary of Defense Chuck Hagel, en route to Asia, said Obama had asked the Pentagon for options on Syria, where an apparent chemical weapons attack that killed as many as 1,000 civilians has upped pressure on Washington to respond.
"The Defense Department has responsibility to provide the president with options for all contingencies," Hagel said. "And that requires positioning our forces, positioning our assets, to be able to carry out different options - whatever options the president might choose." He did not elaborate.
The defense official, who was not authorized to speak publicly, said the USS Mahan, a destroyer armed with cruise missiles, had finished its deployment and was due to head back to its home base in Norfolk, Virginia. But the commander of the U.S. Sixth Fleet has decided to keep the ship in the region, the defense official said.
The official stressed the Navy had received no orders to prepare for any military operations regarding Syria.
Obama's senior national security advisers will convene at the White House this weekend to discuss U.S. options, including possible military action, against the Syrian government, another U.S. official said on Friday.
A senior State Department official said no final decisions were expected from the meeting, pending a further review of intelligence on the attack. Secretary of State John Kerry planned to attend via videoconference. The meeting was expected to take place on Saturday.
The U.S. president has been hesitant to intervene in Syria's 2 1/2-year-old civil war, sentiments he repeated earlier on Friday.
But, in a development that could increase the pressure on Obama, American and European security sources said that U.S. and allied intelligence agencies had made a preliminary assessment that chemical weapons were used by Syrian forces in the attack near Damascus this week.
The security sources, speaking on condition of anonymity, said the assessment was preliminary and, at this stage, they were still seeking conclusive proof, which could take days, weeks or even longer to gather.
Opponents of Syrian President Bashar al-Assad braved the front lines around Damascus to smuggle out tissue samples from victims of Wednesday's mass poisoning,.
The Syrian government denies being responsible and has in the past accused rebels of using chemical weapons, an allegation that Western officials have dismissed.
In his first public comments since Wednesday's attack in the Damascus suburbs, Obama called the incident a "big event of grave concern" and one that demanded U.S. attention, but said he was in no rush to get war-weary Americans "mired" in another Middle East conflict.
Members of Obama's National Security Council, the Pentagon, State Department and intelligence agencies met at the White House late on Thursday, but made no decisions on what to recommend, officials said.
One U.S. official acknowledged that the participants in Thursday's White House meeting aired "differing viewpoints," but rejected the notion that the administration, whose Syria policymaking has been marked by internal dissent in the past, was sharply divided on a response.
"It's not like people were screaming at each other," the official said.
International powers - including Russia, which has long shielded Assad from U.N. action - have urged Assad to cooperate with a U.N. inspection team that arrived on Sunday to pursue earlier allegations of chemical weapons attacks.
White House spokesman Josh Earnest said there was "some evidence" of chemical weapons use in the latest incident, but stopped short of saying an official conclusion was reached.
While the West accused Assad of a cover-up by preventing the U.N. team from visiting the scene, Moscow said the rebels were impeding an investigation.
The United Nations released data showing that a million children were among refugees forced to flee Syria, calling it a "shameful milestone."
CROSSING THE 'RED LINE'
While the preliminary U.S. assessment was that Assad loyalists carried out Wednesday's attack with high-level authorization, one U.S. source closely monitoring events in the region said it was also possible that a local commander decided on his own to use gas to clear the way for a ground assault.
"What we've seen indicates that this is clearly a big event, of grave concern," Obama said in an interview on CNN's "New Day" program that aired on Friday.
Asked about his comment - made a year and a day before the toxic fumes hit sleeping residents of rebel-held Damascus suburbs - that chemical weapons would be a 'red line' for the United States, Obama expressed caution.
"If the U.S. goes in and attacks another country without a U.N. mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it," Obama said. "The notion that the U.S. can somehow solve what is a sectarian complex problem inside of Syria sometimes is overstated."
Obama's caution contrasted with calls for action from NATO allies, including France, Britain and Turkey, where leaders saw little doubt Assad's forces had staged pre-dawn missile strikes that rebels say killed between 500 and well over 1,000 people.
But two years into a civil war that has divided the Middle East along sectarian lines, a split between Western governments and Russia again illustrated the international deadlock that has thwarted outside efforts to halt the killing.
At Thursday's White House meeting, which lasted more than three hours, Obama's aides had a "robust discussion" of the diplomatic and military options available to the president, U.S. officials said.
Among the military options under consideration are targeted missile strikes on Syrian units believed responsible for chemical attacks or on Assad's air force and ballistic missile sites, U.S. officials said. Such strikes could be launched from U.S. ships or combat aircraft capable of firing missiles from outside Syrian airspace, thereby avoiding Syrian air defenses.
NO 'BOOTS ON THE GROUND'
Kerry, who took part in Thursday's meeting by secure video link, advocated the use of air strikes in White House meetings in early June preceding an announcement of military aid to the rebels, a person familiar with the talks said. Joint Chiefs Chairman General Martin Dempsey argued that such a mission would be complex and costly.
The White House on Friday reiterated Obama's position that he did not intend to put "boots on the ground" in Syria, and an administration official said Thursday's meeting also steered clear of the idea of enforcing a "no-fly" zone there.
Another possibility would be to authorize sending heavier U.S. weaponry, such as shoulder-fired anti-aircraft rockets, to the rebels in addition to lighter arms approved in June. But even those limited supplies have yet to start flowing to the rebels.
The top Democrat on the House of Representatives Foreign Affairs Committee urged Obama on Friday to order air strikes against Assad's government.
Representative Eliot Engel cited Obama's statement that the use of chemical weapons by Assad's forces would cross a "red line" and cause the United States to act to halt such violations of international law.
"If we, in concert with our allies, do not respond to Assad's murderous uses of weapons of mass destruction, malevolent countries and bad actors around the world will see a green light where one was never intended," Engel wrote in a letter to Obama and obtained by Reuters.
With Obama's international prestige seen on the line, a former senior U.S. official said the suspected chemical attack was likely to prompt Obama to use limited force, but he did not expect him to try to topple Assad.
"They will feel obliged to do something because the credibility issue is very high here," the former official said, speaking on condition of anonymity.
Obama's failure to confront Assad with the serious consequences he has long threatened would likely reinforce a global perception of a president preoccupied with domestic matters and unwilling to act decisively in the volatile Middle East, a picture already set by his mixed response to the crisis in Egypt.
Obama has shown no appetite for intervention. Polls by Reuters/Ipsos and others have shown that most Americans, weary of wars in Iraq and Afghanistan, are increasingly aware of the Syria conflict but remain opposed to U.S. involvement there.
(Additional reporting by Erika Solomon in Beirut, Mark Hosenball, Arshad Mohammed, Roberta Rampton, Jeff Mason, Andrea Shalal-Esa, Patricia Zengerle, Phil Stewart and Lesley Wroughton; Editing by Warren Strobel and Peter Cooney)