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Reuters contributed to this report. •

WASHINGTON – The US made preparations to attack Syria over the weekend should President Barack Obama choose to do so.On Saturday, US military and national security advisers huddled with Obama at the White House to consider options for responding to the alleged use of chemical weapons by the Syrian government last week.After completing a bus tour through New York and Pennsylvania on Friday, the president returned to a White House that was concentrating on the latest foreign policy crisis.Vehicles pulled up in front of the White House’s West Wing on Saturday morning, bringing officials to the meeting.Secretary of State John Kerry and Secretary of Defense Chuck Hagel, who is in Asia, both participated from outside he capital.The American military updated a list of possible targets to include mobile units of embattled Syrian President Bashar Assad’s army, after extensive and credible evidence surfaced on Wednesday that his forces used chemical weapons against civilians in Ghouta, a suburb of Damascus, killing hundreds.The list already included command and control centers operated by Assad.Hagel said the US Navy moved battleships stationed in the Mediterranean closer to Israel’s coast to close their ranges to Syria after Obama requested “military options” in response to Wednesday’s chemical attack.Existing naval power in the region under the USS Harry S.Truman carrier strike group is extensive. Representing more than 80 armed craft, they give the president the option to fire Tomahawk missiles at Syrian targets without entering the country’s air space.The US Navy will expand its presence in the Mediterranean with a fourth cruisemissile armed warship, a defense official said on Friday.The USS Mahan 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 defense official said.The Pentagon later played down the deployments, insisting the US is not on the brink of a strike. But the moves came after a four-hour Oval Office meeting on Friday with the president and his national security team that focused on possible targets and consequences.The meeting ended with no decision made on whether or not to strike. The US has only made a preliminary assessment that the Ghouta attack involved chemical agents and was perpetrated by the Assad regime, but has made that assessment based on strong circumstantial evidence, officials say.If sarin gas was used, evidence must be collected and analyzed within no more than 10 days. Its presence could be tested for from urine, blood, soil or even clothing samples. The Syrian government has large stockpiles of sarin, and video footage of the attack showed victims suffering from symptoms of the deadly gas.International law bans the use of chemical weapons on any battlefield under any circumstances.And the “responsibility to protect” – a norm agreed upon by global powers at the United Nations 2005 World Summit – compels the international community to respond if a country fails to protect its citizens from genocide, war crimes, ethnic cleansing or crimes against humanity.Obama has faced mounting pressure to act in the days after Ghouta, with international precedent on the use of weapons of mass destruction at stake. The president has said in the past that the US would not tolerate the use of chemical weapons on a mass scale, and has referred to their use in Syria as a “redline.”France has called for “force” if the attack is confirmed to have involved chemical weapons, but Russia, an ally of the Assad regime, has balked at the prospect, charging Syrian rebel forces of conspiring to frame Assad just as United Nations inspectors arrived in Damascus to investigate a separate incident involving chemical weapons.Russia’s stance complicates any path the US would prefer to take through the UN in addressing the Syrian crisis.“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,” Obama told CNN’s Chris Cuomo on Friday morning.Doctors Without Borders said its Damascus facilities treated 3,600 victims with symptoms of neurotoxic exposure, of whom nearly 400 died; the State Department said it had seen the death toll from Ghouta range between 1,200 and 1,800 people.