The United States told its citizens in Yemen on Tuesday to leave the country immediately due to the threat of "terrorist attacks", the US State Department said in a statement.
The State Department also said it had ordered non-essential US government staff in Yemen to leave the country.
The new measures followed a heightened security warning from Washington on Friday that prompted the closure of several Western embassies in Yemen and several US missions across the Middle East and Africa.
It also came after at least four suspected al-Qaeda members were killed in what local tribal leaders said was a US drone strike in central Yemen early on Tuesday.
"The Department urges US citizens to defer travel to Yemen and those US citizens currently living in Yemen to depart immediately," the statement posted on its website said.
"On August 6, 2013, the Department of State ordered the departure of non-emergency US government personnel from Yemen due to the continued potential for terrorist attacks," it added.
Britain said on Tuesday it had withdrawn all staff from its embassy in Sanaa and extended the closure of the mission until further notice.
"Due to increased security concerns, all staff in the British Embassy have been temporarily withdrawn and the Embassy will remain closed until staff are able to return," the Foreign Office said on its website.
"There is a very high threat of kidnap from armed tribes, criminals and terrorists. Be particularly vigilant during Ramadan, when tensions could be heightened," it said.
The BBC's correspondent in Sanaa, said the Yemeni capital has been experiencing unprecedented security measures, with hundreds of armored military vehicles deployed to secure the presidential palace, vital infrastructural buildings and Western embassies in the capital.
The correspondent said that a security source confirmed Yemeni intelligence services had discovered that dozens of al-Qaeda members had arrived in Sanaa over the past few days from other regions in preparation for the implementation of a large plot.
A BBC source described the plot as dangerous, and suggested it was to include explosions and suicide attacks aimed at Western ambassadors and foreign embassies in Yemen, in addition to operations aimed at the Yemeni military headquarters.