Both the US and Britain have closed their embassies in Yemen, with American officials citing threats by the al-Qaida group linked to the failed bombing of a Detroit-bound plane on Christmas Day. In London, Britain's Foreign Office said the embassy in Yemen was closed Sunday for security reasons. She said officials would decide later whether to reopen it on Monday. The confrontation with the terrorist group's branch in Yemen has gained new urgency since the 23-year-old Nigerian accused in the attack, Umar Farouk Abdulmutallab, told US investigators he received training and instructions from al-Qaida operatives in Yemen. President Barack Obama said Saturday that al-Qaida's branch in Yemen was behind the attempt. A message on the US Embassy Web site read, "The US Embassy in San'a is closed today, January 3, 2010, in response to ongoing threats by al-Qaida in the Arabian Peninsula ... to attack American interests in Yemen." An embassy spokesman reached on the phone would not comment if there was a specific threat. On Thursday, the embassy sent a warden notice to American citizens in Yemen urging them to be vigilant and practice security awareness. It was unclear from the statement how long the embassy would be closed. There have been a spate of assaults on the US Embassy in Yemen, the ancestral homeland of Osama bin Laden and the site of the 2000 bombing of the USS Cole. The embassy has closed several times over past threats. The most deadly in recent history happened in September 2008, when gunmen and two vehicles packed with explosives attacked the US Embassy in San'a, killing 19 people, including an 18-year-old American woman and six militants. None of those killed or wounded were US diplomats or embassy employees. Al-Qaida later claimed responsibility. In March 2003, two people were shot dead and dozens more are wounded as police clashed with demonstrators trying to storm the embassy. In March 2008, three mortars missed the US Embassy and crashed into a high school for girls nearby, killing a security guard. Last January, gunmen in a car exchanged fire with police at a checkpoint near the embassy, hours after the embassy received threats of a possible attack by al-Qaida. Nobody was injured. As recently as July, security was upgraded in San'a after intelligence reports warned of attacks planned against the US Embassy. The embassy's closure follows an announcement of US plans to more than double its counterterrorism aid to the impoverished, fragmented Arab nation in 2010 to boost the fight. Gen. David Petraeus, the US general who oversees the wars in Iraq and Afghanistan and who announced the increased aid, visited Yemen on Saturday and met with President Ali Abdullah Saleh, a Yemeni government official said. The official spoke on condition of anonymity because he was not authorized to talk to the press. Yemen has also deployed several hundred extra troops to two mountainous eastern provinces that are al-Qaida's main strongholds in the country and where the suspected would-be Christmas airplane bomber may have visited, security officials said. US and Yemeni investigators have been trying to track Abdulmutallab's steps in Yemen, which he visited from August until Dec. 7. He was there ostensibly to study Arabic in San'a, but he disappeared for much of that time. Al-Qaida has also killed a number of top security officials in the provinces in recent months, underscoring Yemeni government's lack of control of the country. Tribes hold sway in the region, and many of them are discontented with the central government and have given refuge to al-Qaida fighters, both Yemenis and other Arabs coming from Saudi Arabia or war zones in Iraq and Afghanistan. Yemen has carried out a series of airstrikes and raids against al-Qaida hide-outs in nearby provinces last month. The strikes, Yemen's heaviest in years, targeted what officials said were top leaders in the terror network's branch there. But the intensified campaign has not yet reached into the strongholds of Marib and Jouf. Britain has joined the US fight against the Yemeni al-Qaida branch, with the government confirming Sunday that Obama and Prime Minister Gordon Brown agreed to back a counterterrorism police unit in Yemen to tackle the rising terrorist threat from the country in the wake of the failed Detroit-bound plane bombing. "Amongst the initiatives the prime minister has agreed with President Obama is US-UK funding for a special counterterrorism police unit in Yemen," an e-mailed statement, from Brown's Downing Street Office said. Britain is also to host a high-level international conference Jan. 28 to hammer out an international strategy to counter radicalization in Yemen, the poorest in the Arab world. Downing Street said that Britain was already helping to train Yemeni counter-terrorism officials, but a UK government spokeswoman said this is the first time the counter-terrorism police units have been confirmed. In Washington, a senior administration official said American and British forces already provide the Yemeni police counterterrorism assistance, and that he's unaware of any new joint effort that is ready to be announced. The official spoke on the condition of anonymity in order to discuss sensitive matters. Asked to clarify, the UK government spokeswoman said the initiatives in Yemen are part of ongoing work between the UK and the US and that the cost of the counterterrorism police unit would be met by existing funding commitments to Yemen. The spokeswoman said Britain is forecast to give more than 100 million pounds ($161 million) to Yemen in 2011. The spokeswoman, speaking on condition on anonymity in line with UK government policy, was unable to say how long Britain had been working on initiatives in Yemen. Downing Street also said in its statement the prime minister and president believe that in Somalia "a larger peacekeeping force is required and will support this at the UN Security Council." The statement said Britain and the US would support Yemen's coast guard operation. Pirates operating in the waters between Somalia and Yemen have seized four ships in the past week. Brown called last week for a high-level international meeting later this month to devise ways to counter radicalization in Yemen. Downing Street said the government of Yemen had been consulted over the decision to boost the country's coast guard and police operations. The White House said Washington stands ready to work with allies to fight extremism. The official welcomed Brown's move earlier to lead an international conference on Jan. 28 to devise ways to counter radicalization in the country, the poorest in the Arab world. The official also was unable to confirm any plans to push for a larger UN peacekeeping force for Somalia.