US President Barack Obama's top counter-terrorism adviser said Sunday that an active threat from Al-Qaeda led the US to close its embassy in Yemen, where an Al-Qaeda affiliate trained and armed the man accused in the Christmas Day plot against a US airliner. “We're not going to take any chances” with the lives of American diplomats and others at the embassy in Yemen's capital, White House aide John Brennan said. Britain also shuttered its embassy, citing security reasons. Meanwhile, the British government said that Britain and the US will jointly fund a counterterrorism police unit in Yemen amid fears of an increasing threat of international terrorism emanating from the country. Prime Minister Gordon Brown's office said that he and US President Obama agreed to “intensify joint US-UK work to tackle the emerging terrorist threat from both Yemen and Somalia” in the wake of the failed Detroit terror plot. “Amongst the initiatives the PM has agreed with President Obama's US-UK funding for a special counter-terrorism police unit in Yemen,” the statement said. Speaking of the closure of US embassy in Yemen, Brennan said the threat against Americans and Westerners would not ease until Yemen's government got a better handle on the threat from terrorists inside the country. He estimated there are several hundred members of Al-Qaeda in Yemen. Despite the recent focus on the threat from Al-Qaeda in Yemen, Brennan said the US is not opening a new front in the global fight against terrorism. A top counter-terrorism official said Saturday that Al-Qaeda and other extremists are working to test US defenses and launch an attack on American soil. The failed attempt against the plane “is the starkest of reminders of the insidious terrorist threats we face,” said Michael Leiter, director of the National Counter-terrorism Center. “We know with absolute certainty that Al-Qaeda and those who support its ideology continue to refine their methods to test our defenses and pursue an attack on the homeland,” he said. The center, part of the Office of the Director of National Intelligence, draws experts from the CIA, FBI, Pentagon and other agencies who try to ensure that clues about potential attacks are not missed. Officials have said Abdulmutallab's father warned the US Embassy in Nigeria that his son had drifted into extremism in the Al-Qaeda hotbed of Yemen. Abdulmutallab's threat was only partially digested by the US security apparatus and not linked with a visa history showing the young man could fly to the United States. Obama noted that in recent years, the Al-Qaeda affiliate in Yemen has bombed government facilities there as well as Western hotels, restaurants and embassies. An attack on the US Embassy in 2008 killed one American. “So, as president, I've made it a priority to strengthen our partnership with the Yemeni government - training and equipping their security forces, sharing intelligence and working with them to strike Al-Qaeda terrorists,” he said. Downing Street said that Britain is 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 counter-terrorism 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 counter-terrorism police unit will 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. 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.