The United States has issued a global travel alert, citing an al-Qaeda threat that also caused the State Department to announce it would close its embassies this Sunday around the Muslim world.

The advisory prompted Canada's Foreign Affairs Department to release its own warning for travellers and diplomats in the Middle East and North Africa region. The department stated that Canadians are at a reasonably "elevated risk" of attack due to the threat facing a "close friend and ally."

The U.S. State Department warned American citizens on Friday of the potential for terrorism particularly in the Middle East and North Africa, with a possible attack occurring or coming 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 statement said.

Friday's alert warned that al-Qaeda or its allies may target U.S. government or private American interests. It cited dangers involved with public transportation systems and other prime sites for tourists, noting that previous terrorist attacks have centred on subway and rail networks as well as airplanes and boats.

"U.S. citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when travelling," the department said. It recommended that Americans travelling overseas register with consular authorities on a travel registration website.

Scott Stewart, a former special agent with the U.S. State Department, told CBC News that he wouldn't suggest that people cancel their vacation plans, but he had a warning for "Westerners already living in these countries."

"I’d certainly be careful if I were a Canadian or any Westerner living [in the Middle East, North Africa or Afghanistan]," Stewart said in an interview on Friday afternoon. "I would especially stay away from hotels, these big, high-profile hotels that have been hit in the past."

The alert was posted a day after the U.S. announced it would close 21 embassies and consulates this Sunday in the Muslim world because of an unspecified threat. Spokeswoman Marie Harf said the department acted out of an "abundance of caution" and that some missions may stay closed for longer than a day. Sunday is a business day in Muslim countries.

U.S. lawmaker Ed Royce, the Republican chairman of the House foreign affairs committee, said Friday the embassy threat was linked to al-Qaeda and focused on the Middle East and Central Asia.

"We've had a series of threats," Royce told reporters. "In this instance, we can take a step to better protect our personnel and, out of an abundance of caution, we should."

Canadians told to be vigilant

Foreign Affairs Minister John Baird is encouraging Canadian travellers and diplomats in North Africa and the Middle East to exercise added caution following the U.S. travel alert in the region.

For now, Baird says Canada has no plans to close its embassies on Sunday, a regular work day in the region, as the U.S. has done.

"We have been and are in regular contact with our American colleagues. It’s not for me to discuss the nature of the elevated risk that has caused them to take this decision," said Baird in a news release.

"Having said that, obviously when a close friend and ally…has made this determination...it puts us at elevated risk and we are encouraging a higher degree of caution."

Baird says his senior officials are monitoring the situation carefully in conjunction with the U.S. as Sunday approaches.

Several of Canada's missions in the Middle East, including the embassies in Egypt and Saudi Arabia, are closed on Fridays and Saturdays, and normal operating hours are Sunday through Thursday.

The British Embassy in Yemen is to be closed on Sunday and Monday as "a precautionary measure," the Foreign Office has confirmed.

A spokesman would not say if the closure was in response to a specific threat.

The embassy in Sana'a had already been operating with a reduced staff "due to increased security concerns."

List of embassies shut

The U.S. embassies closing their doors on Sunday are located in the following cities: