The United States has issued a global travel alert to warn its citizens of potential "terrorist attacks", particularly in the Middle East and North Africa, a day after it said 22 embassies and consulates would close on Sunday over security fears.
Britain also announced on Friday that it would close its embassy in Yemen for two days - Sunday and Monday. Several embassy staff have been withdrawn from the capital, Sanaa, the foreign office in London said.
A US official told the AFP news agency on Friday that President Barack Obama had ordered "all appropriate steps" to combat a threat that officials said comes from the al-Qaeda armed group.
In a statement released on Friday, the US administration said it had information that al-Qaeda and its affiliates may redouble efforts to carry out attacks in the period between now and the end of August.
"Terrorists may elect to use a variety of means and weapons and target both official and private interests", the US statement read.
"US citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure. Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services."
A spokesman for Britain's foreign office said it was "particularly concerned about the security situation in
the final days of Ramadan and into Eid".
The Muslim fasting month of Ramadan ends next week with the feast of Eid al-Fitr.
The 22 US embassies and consulates to be closed on Sunday were listed on the state department website and are chiefly those in the Muslim world. Closures include Baghdad, Cairo, Sanaa, Dubai and Tripoli.
'Abundance of caution'
The US alert expires on August 31 and state department spokeswoman, Marie Harf, called it "precautionary".
"The Department of State has instructed certain US embassies and consulates to remain closed or to suspend operations on Sunday, August 4," Harf said.
The decision was taken "out of an abundance of caution and care for our employees and others who may be visiting our installations", she said.
Harf said that the embassies would be closed specifically on Sunday, with an assessment afterwards on whether to reopen them.
The US has been especially cautious about security since an attack on its consulate in Benghazi, Libya on September 11 last year.
The attack killed four Americans, including Ambassador Chris Stevens, and led critics in Congress to accuse the state department of insufficient security.