Britons in Yemen have been told to leave the country as the British embassy is to shut down for the weekend over an alleged terrorist threat from al-Qaida.

The Foreign Office said the diplomatic mission in Sana'a will be closed as a "precautionary measure".

However, the Foreign Office did not reveal the nature of any specific threat.

Britain's move follows Washington's decision to close more than a dozen embassies across the Middle East and North Africa.

"The embassy will be closed on 4 and 5 August. We have withdrawn a number of staff from Sana'a, due to increased security concerns. We updated our travel advice to Yemen on 2 August to reflect that," said a Foreign Office spokesperson.

The statement added: "We keep travel advice and the security of our staff and missions under constant review. Our travel advice advises particular vigilance during Ramadan, when tensions could be heightened. We are particularly concerned about the security situation in the final days of Ramadan and into Eid."

While urging Britons to leave the country, the office noted that if the unrest worsens, the diplomatic mission will be unable to extend any help. Widespread clashes have been reported in the Yemeni capital.

The month of Ramadan, which is considered holy by Muslims, ends on 8 August.

The Yemen-based al-Qaida in the Arabian Peninsula (AQAP) is a strong militant outfit capable of launching high-profile attacks in the region.

An American official, speaking on condition of anonymity, told AFP that the threat was "specific, credible, and non-counterable".