WASHINGTON, District of Columbia (AFP) - The United States (US) issued a worldwide warning on Friday that Al-Qaeda may attack in August as it ordered shut its embassies across the Islamic world.
Britain also said it would temporarily close its embassy in Yemen as US lawmakers said the threat likely involved Al-Qaeda's franchise in the country.
"It is much more specific than any of the threat lines that I've seen in the past 10 years," said Representative Peter King, a Republican who serves on the House intelligence committee.
The State Department said attacks were possible "particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula."
"Current information suggests that Al-Qaeda and affiliated organisations continue to plan terrorist attacks both in the region and beyond, and that they may focus efforts to conduct attacks in the period between now and the end of August," the State Department said in a worldwide travel alert for US citizens.
The alert warned of "the potential for terrorists to attack public transportation systems and other tourist infrastructure."
As a precaution, the State Department said it was closing at least 22 US embassies or consulates on Sunday, a work day in many Islamic countries.
The missions cover virtually all of the Arab world and also include two embassies in predominantly Muslim non-Arab nations - Afghanistan and Bangladesh - as well as US-run facilities in Israel.
US State Department spokesman Marie Harf said that some embassies or consulates may decide to remain closed after Sunday.
Britain is closing its embassy in Yemen on Sunday and Monday and has withdrawn staff, the Foreign Office said.
A US official, speaking on condition of anonymity, said that the special alert was issued due to information that was "specific, credible and non-counterable."
Under a policy since the 1988 Pan Am flight bombing over Lockerbie, Scotland, the US is obliged to share with its private citizens any safety information provided to US government employees.
The decision to go public and announce a threat comes as the US government faces criticism over surveillance of citizens, which President Barack Obama says is necessary for security.
The US responded furiously on Thursday after Russia gave asylum to Edward Snowden, a former US contractor who revealed details of US snooping.
In Washington, Republicans have also criticised the Obama administration for not doing more to prevent an attack on Sept 11 last year against the US consulate in Benghazi, Libya.
The assault blamed on Islamist militants killed four Americans, including ambassador Chris Stevens.
Representative Jason Chaffetz, a Republican who has criticized the Obama administration over the Benghazi attack, said he understood there was "a very real worldwide threat."
"There are threats that are coming at us on a daily basis but this rises to a new level," Mr Chaffetz told CNN.
He praised the administration response to the latest threat but called for more counter-terrorism programs.
"We have to take this threat very seriously. And it's not, as the president campaigned on, you know, Al-Qaeda's on the run and... everything's fine," Mr Chaffetz said.
Former secretary of state Hillary Clinton has accepted responsibility for the Benghazi attack but said that requests for more security were only seen by lower-ranking officials.
Under Obama, US forces in 2011 killed Al-Qaeda's fugitive leader Osama bin Laden in a secret raid on his hideout in Abbottabad, Pakistan.
Bin Laden was originally from Saudi Arabia and Al-Qaeda historically focused on the Arabian Peninsula.
Al-Qaeda in the Arabian Peninsula, the global terror network's Yemen-based franchise, has launched a series of foiled attacks on the US in recent years.
Mr Obama on Thursday welcomed Yemen's President Abdrabuh Mansur Hadi to the White House and thanked him for cooperation against Al-Qaeda, including through US drone strikes.
It was unclear if the purported threat was in response to specific events or anniversaries.
Wednesday marks the 15th anniversary of Al-Qaeda's coordinated attacks against the US embassies in Kenya and Tanzania.