The US and Britain locked up their embassies in Yemen after new threats from al Qaida.
The White House expressed alarm at the terror group's expanded reach in the poor Arab nation where an offshoot apparently ordered the Christmas Day plot against a US airliner.
President Barack Obama's top counterterrorism adviser, John Brennan, cited "indications al Qaida is planning to carry out an attack against a target" in the capital, possibly the embassy, and estimated the group had several hundred members in Yemen.
Security reasons led Britain to act, too; it was not known when the embassies would reopen.
The US is worried about the spread of terrorism in Yemen, a US ally and aid recipient, Mr Brennan said, but doesn't consider the country a second front with Afghanistan and Pakistan in the fight against terrorism.
As to whether US troops might be sent to Yemen, Mr Brennan replied: "We're not talking about that at this point at all."
He pledged to provide the Yemeni government with "the wherewithal" to take down al Qaida.
Britain and the US are assisting a counterterrorism police unit in Yemen as fears grow about the increasing threat of international terrorism originating from the country.
The Obama administration claims that the suspect in the plot against the Detroit-bound plane was trained and armed by the al Qaida offshoot in Yemen.
Mr Brennan blamed a series of what he called lapses and human errors in US intelligence and security defences for allowing a Nigerian man to board the plane with explosives. He tried to detonate them as the aircraft approached Detroit on December 25.
