BAQOUBA, Iraq — America’s commander in Iraq has vowed to inflict harsh punishment on the Shiite militia suspected of kidnapping five Britons. As American squads staged raids across Baghdad’s Sadr City, and British officials said they were attempting to contact the hostage-takers, General David Petraeus, the supreme American commander, was reported to have warned that the group responsible for organizing the kidnappings would “be profoundly sorry.”

Sadr City, Baghdad’s vast northeastern Shiite district, is dominated by the Mahdi army of the radical cleric Moqtada al-Sadr. The British computer expert and four security guards seized in the finance ministry building on Tuesday were driven off in the direction of Sadr City.

Yesterday, sources inside Sadr City claimed that the five men had been moved south to the Dayara district. They were said to be in the hands of Abu Daraa, a notoriously violent Shiite warlord and criminal who split with Mr. Sadr to establish his own fiefdom.