"Recent reports indicate that militant groups may be surveilling US citizens for possible kidnapping operations, particularly oil company employees working in Basrah Province, Iraq," the US Embassy in Iraq said in a security message on Wednesday.

"The groups may be focused on US citizens at hotels in the Basrah area.

"We encourage all US citizens to review the existing travel warning for Iraq and take appropriate measures to ensure their safety if travel to Iraq is necessary."

The UK's Foreign and Commonwealth Office said the warning applied equally to British nationals, particularly oil company employees working in Basra province.

Key oil players in the region include Shell, BP, ExxonMobil and Lukoil.

"There is a high threat of terrorism including kidnapping," FCO said.

"There was a marked increase in the number of terrorist attacks across Iraq in 2013.

"While terrorist incidents are less frequent in the Kurdistan region, there have been attacks in September and December 2013 and in January 2014."