Iraq declared Tuesday National Sovereignty Day as U.S. combat troops turned over security duties to Iraqi forces.

On the eve of the hand-off, four U.S. troops died of combat-related injuries, the U.S. military said in a news release issued Tuesday. The soldiers were stationed in Baghdad. Details about the incident weren't released.

On Monday, the U.S. military turned over to the Iraqis control of the one-time defense ministry complex that was the joint base for U.S. and Iraqi forces, USA Today reported.

Speaking at a military parade inside the Green Zone, Iraqi Prime Minister Nouri al-Maliki said, "The national united government succeeded in putting down the sectarian war that was threatening the unity and the sovereignty of Iraq."

Thousands of Iraqis gathered in Baghdad's Zawra Park for a concert to mark the milestone, part of a security agreement that requires all U.S. combat troops to leave Iraq by Aug. 31, 2010.

In Basra, "No No Americans" was spray-painted on walls, The Washington Post reported. Another graffiti artist wrote: "Pull your troops from our Basra. We are its sons and want its sovereignty."

Banners around Baghdad proclaimed: "On the day of sovereignty, we're lighting candles for a better future."

U.S. forces were being deployed to borders and areas around Baghdad and Mosul, where U.S. commanders said they hope troops can interdict militants and weapons.

"We are grateful for what the U.S. military did in toppling Saddam Hussein's regime and fighting the militias and al-Qaida," Iraqi government spokesman Tahseen al-Sheikhly said. "What we need now is their help rebuilding our country."

Under terms of the security agreement, U.S. troops vacated all but a few outposts in Baghdad and are available if Iraqi authorities request assistance, The New York Times reported.