The US state department has issued a travel warning to its citizens worldwide, alerting them to potential "terrorist attacks", particularly in the Middle East, North Africa and the Arabian Peninsula.
In a statement released on Friday, Washington said it possessed information that al-Qaeda armed group and its affiliates may focus efforts to conduct 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 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."
The state department said that the travel alert expires on August 31, 2013.
The statement came a day after state department announced it would close an unspecified number of embassies around the world on Sunday over security concerns.
State department spokeswoman Marie Harf called the step "precautionary" but declined to specify the threat or list which missions would be closed.
'Abundance of caution'
A senior state department official, however, later told reporters they were those that would normally have been open on Sunday, including several US missions in predominantly Muslim countries such as Cairo and Baghdad.
"The Department of State has instructed certain US embassies and consulates to remain closed or to suspend operations on Sunday, August 4," Harf told reporters.
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.
"It is possible we may have additional days of closing as well," she said.
Government offices are nearly always closed in the US on Sunday, which is the start of the work week in many Muslim-majority countries.
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.