WASHINGTON (REUTERS) - The United States was repositioning naval forces in the Mediterranean to give President Barack Obama the option for an armed strike on Syria, although officials cautioned that Mr Obama had made no decision on military action.
A defence official, speaking on condition of anonymity, said on Friday that the US Navy would expand its presence in the Mediterranean to four destroyers from three.
Secretary of Defence Chuck Hagel, enroute to Asia, said Mr 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 Defence Department has responsibility to provide the president with options for all contingencies," Mr 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 defence official, who was not authorised 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 US Sixth Fleet has decided to keep the ship in the region, the defence official said.
The official stressed the Navy had received no orders to prepare for any military operations regarding Syria.
Mr Obama's senior national security advisers will convene at the White House this weekend to discuss US options, including possible military action, against the Syrian government, another US 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 US 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 Mr Obama, American and European security sources said that US 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, Mr Obama called the incident a "big event of grave concern" and one that demanded US attention, but said he was in no rush to get war-weary Americans "mired" in another Middle East conflict.
Members of Mr 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 US 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 Mr Assad from UN action - have urgedMr Assad to cooperate with a UN 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 Mr Assad of a cover-up by preventing the UN 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."
While the preliminary US assessment was that Mr Assad loyalists carried out Wednesday's attack with high-level authorisation, one US 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," Mr Obama said in an interview on CNN's "New Day" programme 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, Mr Obama expressed caution.
"If the US goes in and attacks another country without a UN mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it," Mr Obama said. "The notion that the US can somehow solve what is a sectarian complex problem inside of Syria sometimes is overstated."
Mr Obama's caution contrasted with calls for action from NATO allies, including France, Britain and Turkey, where leaders saw little doubt Mr 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, Mr Obama's aides had a "robust discussion" of the diplomatic and military options available to the president, US 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, US officials said.
Such strikes could be launched from US ships or combat aircraft capable of firing missiles from outside Syrian airspace, thereby avoiding Syrian air defences.
Mr 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 Mr 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 authorise sending heavier US weaponry, such as shoulder-fire.