Military officers said today that U.S. forces in Iraq will begin redeploying Thursday to set up occupation zones as they enter into a postwar phase of enforcing security and restoring services around the country.

The 1st Marine Division, now controlling the eastern side of Baghdad, will withdraw and begin heading south, while the Army's V Corps will cross the Tigris River to take over the entire Iraqi capital, military officials said.

The Marines, aided by British allies, control southeastern Iraq and will take over the entire southern half of the country, while the Army will relieve them in former president Saddam Hussein's home town of Tikrit and occupy the northern half. Eventually, Baghdad will fall under the supervision of the land commander, Army Lt. Gen. David D. McKiernan, who is moving his base to the capital from Kuwait.

The division into three zones will roughly correspond to the tripartite geographic organization set up by retired Army Lt. Gen. Jay M. Garner, who will oversee reconstruction and humanitarian efforts in Iraq. Garner will report to McKiernan, military officials said. While the military will focus on building stability, Garner's fledgling organization, the Office of Reconstruction and Humanitarian Assistance, will take the lead on infrastructure, governance and basic services.

"It's a tremendous responsibility and it's very complex," said Lt. Col. George Smith, a planner for the 1st Marine Expeditionary Force. "We focus the majority of our efforts on war-fighting. That's what we do. And so post-hostilities introduces a whole new spectrum of challenges."

With the last of Hussein's regular army and Republican Guard divisions dismantled, military planners expect to spend much of their time in this next phase countering attacks by remnants of paramilitary groups like Saddam's Fedayeen.

"We see that as the number one threat we're going to have to deal with in post-hostilities," Smith said. "We're going to aggressively hunt these guys down and we'll destroy them."

The 1st Marine Expeditionary Force, with about 60,000 Marines and 26,000 British troops, will take responsibility for nine of Iraq's 18 governates encompassing 9.3 million people and a swath of territory three times as large as Virginia. The Army's V Corps, whose ranks of 50,000 are growing with the arrival of the 4th Infantry Division and other units still en route, will oversee eight governates with about 9 million people, leaving the final district, Baghdad and its 5 million residents, to McKiernan.

Security by itself could be a daunting challenge. Marines patrolling Baghdad today came across two men rushing out of a bank, one armed with an AK-47 assault rifle, and engaged in a fight that killed one of the Iraqis. The Marines found $3.7 million in U.S. dollars and 861,000 Iraqi dinars, worth about $345, according to spokesman Lt. Col. Rick Long. Other Marines raided the Palestine Hotel, home to many journalists, seeking a fugitive who was not identified.

Beyond security, military forces will continue to be occupied with the search for Hussein and his lieutenants, and for the weapons of mass destruction that Hussein's government was accused of hiding. However, they already find themselves chasing down numerous false leads, as it is difficult to distinguish credible tips from unfounded rumors. Marines in Tikrit, for example, pursued a report that Hussein was buried at a palace. But a search of the area turned up nothing.

U.S. Marines restrain a man holding a press card outside the Palestine Hotel in Baghdad. Hundreds of Iraqis tried to enter, claiming to work for foreign media or seeking jobs.