Security forces in Yemen have been placed on high alert amid fears of an al-Qaeda-linked attack.
Foreign diplomatic missions and government offices are being protected by ground troops and tanks.
The move comes as the Obama administration stepped up its drone campaign with a series of four strikes in quick succession, after intercepting messages between two prominent al-Qaeda leaders.
U.S. officials say this intelligence indicated that al-Qaeda’s leader had urged the group’s Yemeni branch to attack Western targets.
Ayman al-Zawahiri, the al-Qaeda leader that replaced Osama bin-Laden is said to have contacted Yemen-based Nasser al-Wuhayshi, head of al-Qaeda in the Arabian Peninsula.
The drone strikes ended a seven-week spell with no U.S. activity in the region, killing four presumed al-Qaeda militants.
As Washington and London evacuate their diplomats from the capital Sanaa in a climate of escalating tension, there are now mounting fears of al-Qaeda liquid bomb attacks.