Five Britons who were kidnapped in Baghdad last month are being held by a secret cell of the Mahdi Army militia that was armed, trained and funded by Iran, the top U.S. commander in Iraq was quoted Thursday as saying.

"A very intensive effort" is under way to find the hostages, Gen. David Petraeus told The Times.

The captives — four security guards and a consultant — were abducted from the Iraqi Finance Ministry on May 29 by some 40 heavily armed men who took them in the direction of Baghdad's sprawling Shiite district of Sadr City.

Iraqi officials have said they believe the Britons were taken hostage by the Mahdi Army militia, which is largely loyal to the radical Shiite cleric Muqtada al-Sadr.

But Petraeus told The Times that "it is a secret cell of Jaish al-Mahdi (the Mahdi Army)."

"They are trained in Iraq, equipped with Iranian (weapons) and advised by Iran," Petraeus was quoted as saying. "The Iranian involvement here we have found to be much, much more significant than we thought before."

Washington has accused Shiite Iran of arming and financing Shiite militias fighting American and Iraqi troops in Iraq — charges Iran denies.

Petraeus said told The Times there have been several operations to try to rescue the hostages but "we just have not had the right intelligence."

Iraqi officials have said the Mahdi Army may have grabbed the men in retaliation for the killing by British forces of the militia's commander in the southern city of Basra.

The British ambassador to Iraq, Dominic Asquith, has appealed to the kidnappers of the five Britons to release them or open negotiations.