Update as of 6:54 a.m. EDT: A suspected U.S. drone killed four alleged al Qaeda members in Yemen on Tuesday while U.S. and British embassies evacuated staff following warnings of a possible attack by al Qaeda, according to an Associated Press report

A drone-fired missile targeted four men in the al-Arqeen district of Marib province, hitting their vehicle and killing all of them, officials said, according to the report. Saleh Jouti, a senior al Qaeda member, may be among the dead, the officials said on condition of anonymity. The drone strike was the fouth in the past two weeks, and three drone attacks have targeted and hit vehicles that transported alleged al Qaeda members in Southern Yemen in recent years.

The U.S. State Department on Tuesday ordered the evacuation of non-emergency U.S. government personnel from Yemen and issued a warning to U.S. citizens to delay traveling to Yemen, while urging those currently in the country to depart immediately, over fears of a possible attack by al Qaeda.

The warning was issued shortly after the State Department on Friday asked U.S. citizens to avoid traveling to the Middle East and North Africa, and urged Americans in these regions to exercise caution until the end of August, over security threats possibly originating in the Arabian Peninsula.

“The security threat level in Yemen is extremely high,” the State Department said in a statement, adding, “In September 2012, a mob attacked the U.S. Embassy compound. Demonstrations continue to take place in various parts of the country and may quickly escalate and turn violent. U.S. citizens are urged to avoid areas of demonstrations, and to exercise extreme caution if within the vicinity of a demonstration.”

Security measures have been tightened in Yemen’s capital, Sanaa, where hundreds of army vehicles have been deployed to secure the presidential palace, BBC reported on Tuesday.

Yemeni intelligence officials have found that dozens of al Qaeda members entered Sanaa in recent days, believed to be part of an attack plan, the report said. The plot, reportedly involving bombings and suicide attacks, could target ambassadors representing Western governments and foreign embassies, according to Yemeni officials.

The State Department said terrorist organizations, including al Qaeda in the Arabian Peninsula, have been active in Yemen, adding that the U.S. government remains “highly concerned” about potential attacks on U.S. citizens, U.S. government and private establishments, and targets perceived as serving U.S. and Western interests.

The State Department said the government will assist evacuation “on a cost-recovery basis,” which means the traveler is liable to reimburse the U.S. government for travel costs, but it added that the government does not have plans, at present, to sponsor departures.

Americans who choose to stay in Yemen were urged to limit travel within the country and to prepare for any contingencies.