(Updates with White House statement after meeting ends)
By Roberta Rampton and Jeff Mason
WASHINGTON, Aug 24 (Reuters) - U.S. military and national security advisers presented President Barack Obama on Saturday with a range of options for responding to the alleged use of chemical weapons by the Syrian government, the White House said.
Obama met with top advisers including Vice President Joe Biden, national security adviser Susan Rice, and CIA Director John Brennan to hash out responses.
Obama has been reluctant to intervene in Syria's civil war, which he has described as a "sectarian complex problem." But he said a year ago that chemical weapons would be a "red line" for the United States.
Syrian opposition accounts that between 500 and well over 1,000 civilians were killed this week by gas in munitions fired by pro-government forces, and video footage of victims' bodies, have stoked demands abroad for a robust, U.S.-led response after 2-1/2 years of international inaction on Syria's conflict.
The White House said Obama spoke with British Prime Minister David Cameron about Syria on Saturday and both men expressed "grave concern" about the reports of chemical weapons use.
"In coordination with international partners and mindful of the dozens of contemporaneous witness accounts and record of the symptoms of those killed, the U.S. intelligence community continues to gather facts to ascertain what occurred," the White House said in a statement.
"The president also received a detailed review of a range of potential options he had requested be prepared for the United States and the international community to respond to the use of chemical weapons."
After completing a bus tour through New York state and Pennsylvania on Friday, the president returned to a White House that was concentrating on its latest foreign policy crisis.
Vehicles pulled up in front of the West Wing on Saturday morning, bringing officials to the meeting. Secretary of State John Kerry, who is on vacation in Massachusetts, and Secretary of Defense Chuck Hagel, who is traveling in Asia, both participated remotely.
The White House declined to say when the meeting ended, although it appeared to have lasted about three hours. Director of National Intelligence James Clapper was seen exiting the building around 12:45 p.m. (1645 GMT).
Other participants included Samantha Power, the U.S. representative to the United Nations; Chairman of the Joint Chiefs of Staff Martin Dempsey; and Deputy Attorney General James Cole.
Administration officials were cautious in describing the content of the discussions.
"We have a range of options available, and we are going to act very deliberately so that we're making decisions consistent with our national interest as well as our assessment of what can advance our objectives in Syria," a White House official said before the meeting on Saturday.
Obama said in a CNN interview broadcast on Friday that the United States was still gathering information about the apparent chemical weapons attack that killed as many as 1,000 civilians in a Damascus suburb.
He noted, however, that chemicals weapon use on a large scale would start "getting to some core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region."
American and European security sources have said U.S. and allied intelligence agencies made a preliminary assessment that chemical weapons were used by Syrian forces in the attack.
Before the Saturday meeting, officials warned not to expect a decision until concrete intelligence assessments were made.
"Once we ascertain the facts, the president will make an informed decision about how to respond," a White House official said.
The United States is repositioning naval forces in the Mediterranean to give Obama the option for an armed strike on Syria, although officials have cautioned that Obama had made no decision on such 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.
Hagel said on Friday that Obama had asked the Pentagon for options on Syria.
"The Defense Department has responsibility to provide the president with options for all contingencies," Hagel said en route to Asia. "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.
Syrian state television said soldiers found chemical materials on Saturday in tunnels that had been used by rebels, rejecting the blame for carrying out a nerve gas attack.
The Syrian government denies being responsible for the attack and has in the past accused rebels of using chemical weapons, an allegation that Western officials have dismissed.
Iranian President Hassan Rouhani weighed in on the issue for the first time, saying chemical weapons had killed people in Syria, its ally. Although Rouhani stopped short of saying who he thought had used the weapons, Iran's Foreign Ministry said evidence pointed to rebels fighting Syrian President Bashar al-Assad.
The U.S. Central Command and the Jordanian armed forces were planning to host a meeting of regional defense chiefs from Sunday to Tuesday in Jordan. The group will discuss "the region's dynamic security environment." The meeting was scheduled in June and not called in response to the recent attacks in Syria, a Pentagon spokesman said. (Additional reporting by Phil Stewart, Lesley Wroughton, Andrea Shalal-Esa and Paul Eckert; Editing by Peter Cooney)