carry out an attack as early as last Sunday.

CNN reported that Zawahiri told Wuhayshi to "do something", causing officials in both Washington and Yemen to fear an attack was imminent.

As a result, roughly two dozen US diplomatic posts were shut and the State Department said 19 would remain shut through Saturday.

AQAP is seen as the terrorism network's most capable franchise following the decimation of its core leadership in Afghanistan and Pakistan in recent years.

The Yemen-based group has attempted a number of attacks on US soil, including a bid to bring down a passenger plane in 2009 by a man wearing explosives in his underwear and a failed plot to send bombs concealed in printers.

The United States in turn has launched scores of drone strikes in Yemen, where themilitant group thrives in vast, lawless areas largely outside the government's control.

Tribal source reports drone strike on militants

A drone strike in Yemen early on Tuesday struck a vehicle, killing four suspected Al Qaeda militants "in a ball of fire," a tribal source told AFP.

One of the four people killed was on a list released by Yemeni authorities of 25 Al Qaeda operatives suspected of plotting attacks to coincide with the end of the Muslim holy month of Ramadan, according to the source.

It was not immediately clear if the State Department alert was related to the drone strike. US officials, who rarely acknowledge the covert drone program, could not be reached for comment.

Several US allies, including Britain, France, Germany and Norway, have also announced closures of some of their