The US State Department has issued a worldwide travel warning to US citizens lasting throughout the month of August, exhorting travellers to be on alert for possible terrorist attacks on public transport and tourist destinations.

The unusually broad warning, that covers travel by Americans anywhere in the world until 31 August, comes just a day after the US government announced that it was closing at least 18 embassies on Sunday in what it described as a "precautionary measure" tied to evidence of plots emanating from the Middle East and North Africa. On Friday it was announced that the British embassy in Yemen is also to close for two days.

CNN reported that the US travel alert related to intelligence received by officials about a potential attack originating in Yemen. It said its information was based on three sources, who said the US had information that al-Qaida in Yemen was "in the final stages of planning for an unspecified attack".

The State Department travel alert says: "Current information suggests that al-Qaida 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."

"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."

Seth Jones, a terrorism expert at the Rand Corporation, said the combination of the closure of US embassies and the worldwide travel alert was unusual and significant. It came on the back of recent active threats to US embassies in Libya and Egypt and in Gulf states including Yemen, he said.

Jones, who testified before the House foreign affairs sub-committee on terrorism last month, said he would be very surprised if the threats were co-ordinated across countries and continents by al-Qaida, which had generally lost its ability to act across national boundaries since 9/11 and had been weakened at its core in Pakistan. But equally the confluence of embassy closures and the travel alert pointed to the enduring potency of splintered al-Qaida affiliates around the world.

"When you start shutting down your embassies because of the threat of terrorist attack, and when you put out a worldwide travel alert, that suggests that al-Qaida affiliates are not on the verge of strategic defeat," Jones said.

The embassy closures were taken, a Department of State official said, out of an "abundance of caution and care for our employees". It related to all overseas US diplomatic posts that open on Sundays, though the action could keep embassies closed for several days through the end of Ramadan.

US outposts are likely to remain alert as the anniversary of the attack on the consulate in Benghazi in Libya approaches on 11 September. Four Americans, including the ambassador Chris Stevens, were killed in the attack after which Republicans accused the Obama administration of failing to protect US diplomats in sensitive places.