The US government has flown diplomatic staff out of Yemen and is urging its citizens to leave "immediately" amid concerns over security threats.
It follows the sudden closure of 20 US embassies and consulates on Sunday.
This was prompted by intercepted conversations between two senior al-Qaeda figures, including top leader Ayman al-Zawahiri, US media said.
Meanwhile the UK Foreign office says it has withdrawn embassy staff from Yemen and advises against all travel there.
The US earlier said the closures in North Africa and the Middle East were "out of an abundance of caution".
A global travel alert issued on Tuesday said: "The US Department of State warns US citizens of the high security threat level in Yemen due to terrorist activities and civil unrest.
"The department urges US citizens to defer travel to Yemen and those US citizens currently living in Yemen to depart immediately."
It added that "the security threat level in Yemen is extremely high".
The Pentagon said the US Air Force was flying staff out of the capital Sanaa on Tuesday morning "as part of a reduction in emergency personnel".
It comes hours after a drone strike reportedly killed four suspected al-Qaeda militants in the country.
"The US Department of Defense continues to have personnel on the ground in Yemen to support the US state department and monitor the security situation.
The BBC's Abdullah Ghorab, in Sanaa, says the Yemeni capital has been experiencing unprecedented security measures, with hundreds of armoured military vehicles deployed to secure the presidential palace, vital infrastructural buildings and Western embassies in the capital.
Our correspondent says that a security source confirmed that 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.
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.
Both the White House and the US state department have said the current threat comes from al-Qaeda in the Arabian Peninsula (AQAP), but have refused to divulge further details.
