The US has indications that al-Qaeda is planning an attack in the Yemeni capital Sanaa, President Barack Obama's top counter-terrorism adviser has said.
John Brennan was speaking after the US and UK announced their embassies in Sanaa had temporarily closed.
The US has accused a Yemen-based offshoot of al-Qaeda of being behind the alleged Christmas Day bomb attempt on a US jet flying to Detroit.
There are mounting fears that Yemen is becoming a leading al-Qaeda haven.
Mr Brennan, the US deputy national security adviser for homeland security and counterterrorism, told ABC the group had "several hundred members" in Yemen and was posing an increasing threat there.
"We know that they have been targeting our embassy, our embassy personnel," he said.
Speaking separately to CNN, he said there were indications that a radical US cleric of Yemeni origin had links both to the Christmas Day bomber and the man accused of the Fort Hood shooting massacre.
Links to cleric
Last week an organisation called al-Qaeda in the Arabian Peninsula urged Muslims to help in "killing every crusader who works at their embassies or other places".
In an internet statement, the group also said it was behind an attempt to bomb a transatlantic airliner on Christmas Day.
On Saturday, President Barack Obama said the organisation appeared to have trained 23-year-old Nigerian accused Umar Farouk Abdulmutallab, who is being held in a US prison.
Mr Brennan said there were "indications" that Mr Abdulmutallab had had direct contact with radical cleric Anwar al-Awlaki, who has been on the run in Yemen since December 2007.
It was clear, he said, that Mr Awlaki had also been in touch with Nidal Malik Hasan, the US army major charged with killing 13 people at Fort Hood last year.
It was not immediately clear when the UK or US embassies in Sanaa would reopen.
In a statement on its website, the US embassy said it would be closed on Sunday "in response to ongoing threats by al-Qaeda in the Arabian Peninsula to attack American interests in Yemen".
The embassy also reminded US citizens in Yemen to be aware of security.
A Foreign Office spokeswoman said the British embassy was closed on Sunday and a decision would be taken later on whether to open it on Monday.
Hours earlier, UK Prime Minister Gordon Brown told the BBC: "This is a new type of threat and it is from a new source which is obviously Yemen, but there are many other potential sources Somalia, as well as Afghanistan and Pakistan."
The US mission in Sanaa was the target of an attack in September 2008, which was blamed on al-Qaeda, and in which 19 people died, including a young American woman.
Also on Saturday, Gen David Petraeus, head of US military operations in the Middle East and Central Asia, visited Yemeni President Ali Abdallah Saleh to pledge US support for its fight with al-Qaeda.
The visit came a day after the general announced that the US would more than double counter-terrorism aid to Yemen this year.
The US provided $67m (£41m) in training and support to Yemen last year; only Pakistan receives more, with about $112m, according to AP news agency.
Yemeni officials said on Saturday they had sent more troops to fight al-Qaeda militants in the provinces of Abyan, Baida and Shabwa.
"These measures are part of operations to hunt down elements of al-Qaeda... and tighten the noose around extremists," a Yemeni official told AFP news agency.
Analysts say the US has also provided intelligence to Yemeni forces, which carried out raids last month that reportedly left dozens of militants dead.
In his weekly address on Saturday, President Obama said militant training camps in Yemen had already "been struck, leaders eliminated, plots disrupted".
Correspondents say the security situation in Yemen is complicated by an abundance of firearms, an insurgency in the north and a secessionist movement in the south.
But the prospects of re-asserting central government authority over the lawless areas where al-Qaeda is based look, in the opinion of some analysts, remote - even with beefed-up American support.