The United States has told citizens and non-emergency government staff to leave Yemen "immediately" over fears of a possible terrorist attack.
The State Department said there is a high threat level in the Arab state due to terrorist activities and civil unrest.
The warning came after at least four suspected al-Qaeda members were killed in central Yemen in what tribal leaders said was an American drone strike early on Tuesday.
Late last week the US announced it would close 20 of its embassies and consulates on Sunday, prompted by intercepted conversations between two senior al-Qaeda figures, including top leader Ayman al-Zawahiri, US media said.
The US earlier said the closures in North Africa and the Middle East were "out of an abundance of caution".
The BBC reports the Yemeni capital of Sanaa has been experiencing unprecedented security measures, with hundreds of armoured military vehicles deployed to secure the presidential palace, vital infrastructural buildings and Westerns embassies in the capital.
Its correspondent says that a security source confirmed Yemeni intelligence services had discovered that tens 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.
The 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.
The White House and the US State Department have said the current threat comes from al-Qaeda in the Arabian Peninsula, but have refused to divulge further details.
A number of US diplomatic posts in the region - including in the Yemeni capital - will remain closed until Saturday.