The security threat in Yemen has been moved to “extremely high” after a bugged telephone conversation between al Qaeda leaders disclosed a plot for a major attack.

The UK and the US State Department have instructed that their embassies should be evacuated and all non-emergency US government staff have been ordered to leave the country “due to the continued potential for terrorist attacks”.

US citizens have been told to leave the country “immediately”.

A statement from the UK Foreign Office said: “Due to increased security concerns, all staff in our Yemen embassy have been temporarily withdrawn, and the embassy will remain closed until staff are able to return.”

The travel warning came just hours after it was disclosed that a five missile-drone strike had killed four alleged al Qaeda members.

The hit in Yemen’s central Marib province targeted a vehicle turning it into “a ball of fire”, according to Yemeni officials.

Speaking on condition of anonymity because they are not allowed to talk to the media, they said they believed that one of the dead is Saleh Jouti and another Saleh al Tays al Waeli.

Waeli was on a list of 25 “most wanted” al Qaeda operatives included in a list released by the Yemen government on Monday.

The men are wanted in connection with an alleged plot to launch a major attack before Ramadan ends and the Eid al-Fitr feast begins, either on Thursday of Friday.

The developments in Yemen came as it was disclosed that an intercepted telephone call between al Qaeda leaders triggered the terror alert that led to the temporary closure of 19 US diplomatic posts across the Middle East and Africa.

During the conversation, al Qaeda chief Ayman al Zawahri instructed the head of the terror affiliate in Yemen, Nasser al Wuhayshi to carry out a major attack as early as last Sunday.

The plot is thought to have been one of the most serious against American and other Western interests since the September 11, 2001, attacks, according to US intelligence officials.

One American official, who had been briefed on the intelligence report, told the New York Times: “This was significant because it was the big guys talking, and talking about very specific timing for an attack or attacks.”

Al Wuhayshi, head of Al Qaeda in the Arabian Peninsula (AQAP), is a significant player and was groomed for leadership in the multi-headed terror organisation by Osama bin Laden.

Tuesday’s drone strike is the fourth of its kind since July 28. The raids have killed 17 suspects in one week.

Al-Qaeda in the Arabian Peninsula is seen as the terror 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 the militant groups thrive in vast, lawless areas largely outside government control.

Several US allies, including Britain, France, Germany and Norway have also announced closures of some of their missions in the region.

A Sept. 11 attack last year killed the US ambassador to Libya, Christopher Stevens, and three other Americans in Benghazi.

US Republican Senator Lindsey Graham told CNN at the weekend that the recent actions taken to close the embassies showed President Barack Obama’s administration had learned lessons from Benghazi.

[adrotate group=”15″]