The United States issued a worldwide alert Friday warning of plans by Al-Qaeda to launch an attack in the Middle East or North Africa in August.
The State Department issued the caution to US citizens a day after announcing that some two dozen embassies or consulates would be closed on Sunday as a precaution.
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 organizations 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.
The missions posted notices on their websites saying that they would be closed on Sunday.
US State Department spokeswoman Marie Harf said that some embassies or consulates may decide to remain closed after Sunday.
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 United States is obliged to share with its private citizens any safety information provided to US government employees.
Nancy Pelosi, the leader of President Barack Obama's Democratic Party in the House of Representatives, said that congressional leaders received a briefing on the threat.
"There is some understanding of the seriousness of the threat," Pelosi told reporters.
Representative Jason Chaffetz, a Republican who has criticized the Obama administration over embassy security, 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," Chaffetz told CNN.
The United States has been especially cautious about security since an attack on its consulate in Benghazi, Libya on September 11 last year.
The assault blamed on Islamist militants killed four Americans, including ambassador Chris Stevens.
Republicans have accused the Obama administration of failing to ensure adequate security at the Benghazi consulate.
Chaffetz said that the latest threat showed the need 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," 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 (AQAP), the global terror network's Yemen-based franchise, has launched a series of foiled attacks on the United States in recent years.
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.