Colonel Butch Kievenaar, the senior US military commander in southern Iraq, said in a defence department briefing earlier this week that some Iraqi fighters who were believed to have gone to Iran had recently returned to the Basra region.
Targets of violence
He said he believed they were pursuing criminal activities such as extortion, but there had been no clear sign that they had a "militia" agenda.
Violence had dropped throughout the country in recent months, but in the run-up to the withdrawal of US combat troops from the centre of cities and towns on June 30, attacks increased.
In another attack in the capital, Baghdad, at least two Shia pilgrims were killed by a roadside bomb as they travelled to a shrine marking the death of a revered imam.
Police and medical officials said that at least 12 other people were injured by the bomb, which was planted under a bridge in the Jadida neighbourhood in the east of the city.
About 100,000 police officers have been deployed in Baghdad to try to prevent attacks against the hundreds of thousands of pilgrims, who have previously been regular targets of violence.
At least 12 other pilgrims were wounded in two other separate bombings elsewhere in the capital on Friday.