WASHINGTON - US President Barack Obama's top national security advisers met at the White House on Saturday to discuss the potential threat of terrorist attacks that caused Washington and its allies to issue travel warnings and close embassies throughout the Middle East.The United States issued a worldwide travel alert on Friday warning Americans that al Qaeda may be planning attacks in August, particularly in the Middle East and North Africa.Without giving an update on the nature of the threat, the White House said top officials including Secretary of State John Kerry, Secretary of Defense Chuck Hagel, and national security adviser Susan Rice gathered on Saturday to discuss it."Early this week, the president instructed his National Security team to take all appropriate steps to protect the American people in light of a potential threat occurring in or emanating from the Arabian Peninsula," the White House said in a statement."This afternoon, National Security Advisor Rice chaired a meeting with the Principals Committee to further review the situation and follow-up actions," it said, referring to the group of advisers.A senior U.S. official, asked if the United States had pre-positioned forces to deal with the latest threat, replied, "We've had US forces prepared for some time to respond to potential contingencies in the Middle East and North Africa."We're postured to support timely and effective action if requested. This latest threat is serious, and the Pentagon is working closely with its partners, to include the State Department and the intelligence community, to confront it," the official said on condition of anonymity.Obama is spending the weekend at the Camp David presidential retreat after playing golf earlier on Saturday. His birthday is Sunday. The White House said Obama had received regular briefings about the potential threat and US preparedness measures all week. Rice and counter-terrorism adviser Lisa Monaco briefed him after the high-level meeting on Saturday, it said.Other attendees at the White House meeting included Obama's chief of staff, Denis McDonough, Chairman of the Joint Chiefs of Staff Martin Dempsey, Homeland Security Secretary Janet Napolitano, Director of National Intelligence James Clapper, and the heads of the FBI and CIA.The US State Department said on Thursday that American embassies that would normally be open this Sunday - including those in Abu Dhabi, Baghdad and Cairo - would be closed.CBS News reported that intelligence officials had information about a major plot."Intelligence officers have reporting from a reliable source that a major plot is under way and that the team to carry it out has been selected and is in place," CBS reported. US authorities did not know the date, the timing or the target of the planned attack, it said.France said earlier on Saturday it would close its embassy in Yemen for several days from Sunday, following similar moves by Britain and Germany.