A day after President Obama's top national security aides huddled at the White House, senior lawmakers appeared on television on Sunday with ominous warnings about intelligence "chatter," first revealed on Thursday, similar to what American spy agencies picked up in the weeks before the 9/11 attacks.
American officials say they have no clear information about where they should try to defend against the attack.
"The assumption is that it's probably most likely to happen in the Middle East," said Representative Peter T King, who is a member of the House Intelligence Committee, on the ABC News program "This Week."
The one aspect of the intelligence that officials appear to agree on is that al-Qaeda's affiliate in Yemen is behind the plotting.
The State Department said Sunday that it was extending the closing of 19 diplomatic posts in the Middle East and North Africa through at least till August 10 because of continued fears of an imminent attack. Several European countries have also closed embassies in the Middle East.
The embassies that will be closed include ones in Yemen, Libya, Egypt, Saudi Arabia and the UAE, the State Department said.