The US State Department has issued a worldwide travel alert to all US citizens because of a potential al-Qaida terror threat.
The department said the potential for terrorism was particularly strong in the Middle East and North Africa and an attack could occur or come from the Arabian Peninsula.
Officials said the unspecified attack could take place at any point until the end of August. The alert will end on 31 August, with the US State Department advising US citizens travelling abroad to be vigilant
The alert comes a day after officials said the US will be closing its embassies and consulates throughout the Muslim world on Sunday after receiving an unspecified threat.
A US official said the alert was based on the same intelligence information that prompted the embassy closures. Sunday is a working day in the Muslim world.
Officials said they were acting out of an "abundance of caution". At least 18 diplomatic posts will be closed because of the threat and could remain so for some time.
The department said: "Terrorists may elect to use a variety of means and weapons and target both official and private interests.
"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.
"US citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling."
John Bolton, former US ambassador to the UN said the alert indicated the US government must have some "pretty good information" about a possible threat.
The US consulate in Benghazi, Libya, was attacked last year on 11 September, which left four Americans dead.
The department added: "We continue to work closely with other nations on the threat from international terrorism, including from al-Qaida.
"Information is routinely shared between the US and our key partners in order to disrupt terrorist plotting, identify and take action against potential operatives, and strengthen our defences against potential threats."
The Foreign Office has updated its travel advice for Saudi Arabia, Oman, Qatar, Bahrain, Kuwait, UAE and Iraq.
It says: "US Embassies in the region have been instructed to close on Sunday 4th August. The British Embassies remain open but staff have been advised to exercise extra vigilance as we approach Eid."