Britain, the US and three other European nations either kept their embassies in Yemen closed or restricted public access amid fears of an attack by the local wing of al-Qaida, which has been blamed for trying to blow up an airliner on Christmas Day.
The British and American embassies in Yemen's capital, Sana'a, remained closed for a second day. A state department spokesman said the situation would be assessed day by day, based on a perceived threat to US personnel. The Foreign Office said it was reviewing the situation.
France's foreign ministry said its embassy in the city had also been shut to the public since yesterday, although staff are still working in the building. Spain and Italy had also restricted public access to their missions, officials said.
The closures came as the BBC quoted reports from the country saying the security alert had been caused when Yemen's military lost six trucks containing explosives and other armaments.
Yemen said today its soldiers had killed two al-Qaida militants during fighting in the Arhab region, north-east of Sana'a. Last month saw a series of government raids in the region against suspected al-Qaida cells.
Yemeni government officials, speaking anonymously to the Associated Press, said security forces had attacked al-Qaida militants including Nazeeh al-Hanaq, a senior figure on Yemen's most wanted list, as they moved through the mountainous area. Hanaq escaped, but two others with him were killed, they said.
While AP said the fighting was not directly linked to the embassy closures, Reuters quoted another official as saying the militants targeted "are believed to be behind the threats directed to the US embassy".
Officials in London and Washington said the decision to close the missions was a result of specific intelligence. John Brennan, the US counterterrorism chief, said the American embassy, which was attacked twice in 2008, was shut because of "indications al-Qaida is planning to carry out an attack against a target inside of Sana'a – possibly our embassy. We're not going to take any chances with the lives of embassy personnel," he said.
The embassy closures came a day after the top US commander for the Middle East, General David Petraeus, visited Yemen to pledge support from Barack Obama for the government's campaign against al-Qaida and a doubling of America's $67m (£41.5m) counterterrorism assistance.
Gordon Brown told the BBC yesterday that "we've got to do more" to combat terrorism in Yemen and called for a conference in London at the end of the month to discuss what more the international community could do to contain the growing al-Qaida threat from there.
Brown said Britain would step up its own support for counter-terrorist units and Yemeni coastguard operations, but Downing Street later said the money would be allocated from within the existing Yemen aid budget.
The US has been training Yemeni counterterrorist forces for several years and Petraeus said that support would be doubled.
But experts on the region said without more help Yemen risked becoming a failed state, similar to Somalia. Ginny Hill, a specialist on Yemen at the Chatham House thinktank, said the attacks were "a symptom of a much deeper underlying problem, the collapse of the state system".
She said Yemen was reliant on oil revenues which are expected to dwindle to nothing over the next decade. "It's not as if you're starting at a static position, you're working on a downward trajectory," she said.
Over the weekend, Obama named al-Qaida in the Arabian Peninsula as the group behind the attempt by Umar Farouk Abdulmutallab on Christmas Day to bomb a commercial airliner landing in Detroit. Claiming responsibility last week, the al-Qaida affiliate urged Muslims to start "killing every crusader who works at their embassies or other places".
Amid the heightened al-Qaida threat against western targets, several international aid and development agencies working in Yemen have begun to restrict their operations and tighten security.
Naseem ur-Rahman, a spokesman for Unicef, the UN children's fund which is providing aid to tens of thousands of impoverished families displaced by the war against the northern Huthi rebels, said the agency had been working with a reduced staff in Sana'a since the heightened security threats last month.
"All our frontline UN staff are still in place and we will not move any until we receive a signal from the government," said Rahman. "But we are working with a reduced staff in Sana'a because of the latest security concerns."
Andrew Moore, the British director of Save the Children in Yemen, said his agency had pulled out all foreign staff from their office in Abyan, the scene of a US-backed air strike on 17 December aimed at al-Qaida leaders, but which local sources said killed as many as 50 people, including women and children.
Moore said staff were avoiding high profile restaurants and hotels in Sana'a, were travelling in unmarked cars and had recently hired an international security consultant to travel to Yemen to oversee their operation.
"There's quite a lot of anger in Abyan and this is a nervous time," said Moore. "The expectation is that al-Qaida will try and do something big in revenge [for the Yemeni attacks]. The big question is, will they try hit a soft target."
Both American and British embassies have been targeted in recent years. In 2008, 19 people were killed by a bomb attack on the US embassy while an al-Qaida plot to bomb the British mission was foiled three years earlier.
Since then, security has been stepped up dramatically. Perched on a hill in northern Sana'a, the British embassy is surrounded by blast walls, security cameras and razor wire, while its main buildings are dug deep into the hillside, meaning key offices are underground.
A few miles away, the US embassy is also heavily guarded, set back from the main road and surrounded by heavily fortified checkpoints with blast walls, ramps and several layers of heavily armed security.