About 20 US tanks and other military vehicles took up positions around Tahrir Square at the very centre of Baghdad on the east bank of the Tigris River, a Reuters correspondent said.

"I can see about 15 tanks and armoured vehicles as well as some other vehicles that appeared to have crossed the river from the west of the city," Hassan Hafidh said from a vantage point near the square, where the Liberation Monument marks, for many, the very heart of the sprawling Iraqi capital.

"They are broadcasting messages in Arabic from loudspeakers telling people not to shoot at them. The Americans look a bit nervous and are pointing their guns at people in the street."

An advance across the nearby Jumhuriya Bridge over the Tigris would all but close a pincer movement that involves the US Third Infantry Division coming up from Kuwait and through southwestern Baghdad over the past three weeks, meeting with US marines who have advanced on Baghdad from other directions.