"Tensions remaining from the recent events in Iraq may increase the potential threat to US citizens and interests abroad, including by terrorist groups," the State Department said in a statement.

"The US government remains deeply concerned about the security of US citizens overseas," it said in a "worldwide caution" that renewed an existing warning issued on March 19 just hours after the Iraq war began.

"US citizens are encouraged to maintain a high level of vigilance and to take appropriate steps to increase their security awareness," it said.

The threats noted in the statement were suicide operations, kidnappings and bombings including attacks that might use non-conventional weapons such as chemical or biological agents.

It also stressed that terrorists and other extremist groups were now likely to target civilian facilities including residential neighborhoods, private clubs, restaurants, places of worship, schools, hotels, outdoor recreation events, resorts and beaches.

"If such facilities cannot be avoided, US citizens should increase their security awareness at such locations," the department said.

The global warning, which is not set to expire until September, was issued just five days after the US Department of Homeland Security lowered the domestic terrorist attack alert from "high" to "elevated."

The department had raised the alert to its second-highest level on March 17, just before the start of the Iraq war, over fears the conflict would spark terrorist reprisals.

The nationwide alert system was introduced after the September 11 attacks on New York and Washington in 2001 blamed on Osama bin Laden's al-Qaeda network.

Wednesday's switch was the third time in the past year that the national alert level has been raised and then brought down again.

SPACE.WIRE