An Iraqi missile hit a tactical operations centre for the US Army's 2nd Brigade of the 3rd Infantry Division in the southern outskirts on Baghdad on Monday.

Two soldiers and two journalists were killed and 15 people wounded in the attack.

Two of those wounded were in critical condition, a US military official told Reuters.

On the eastern outskirts of Baghdad, two Marines were killed while trying to secure two bridges over the Nahr Diyala tributary of the Tigris river.

"It was friendly fire. The artillery came in short," a Marine told Reuters. "The Iraqis have been firing little artillery."

Reuters reporter Matthew Green said the Marines later secured the bridges and crossed with tanks and armoured vehicles. They were now battling Iraqi Special Republican Guard units on the Baghdad side of the river, he said.

Colonel B P McCoy told Reuters the cause of the deaths was unclear, but he added that the Marines were investigating.

Earlier on Monday, troops of the 2nd Brigade burst into the heart of Baghdad and seized two of President Saddam Hussein's palace complexes.