"This wasn't a patrol -- go in and come out," Navy Captain Frank Thorp said here at US Central Command's forward planning base.

"This is part of the (US Army's) Fifth Corps moving into the city."

A US field commander in Baghdad earlier reported that some 30 US tanks rolled deep into Baghdad early Saturday, marking a new step in the drive to oust President Saddam Hussein.

Areas of heavy fighting were reported as a tank battalion task force from the US Army's 3rd Infantry Division, part of the Fifth Corps, punched its way along a road from the airport, 20 kilometers (12 miles) to the southwest.

"We've moved pretty much into the middle of the city," Thorp said. "What we have are elements of the Fifth Corps who moved up from Karbala a couple of days ago and have been south of the city and have now moved up into the city."

He said "significant numbers" of US forces were taking part in the operation and had encountered "sporadic resistance" from what he described as remnants of the Al-Nida and Baghdad Divisions of the elite Republican Guard Corps.

"We had the opportunity and we moved in," Thorp explained, declining to say what the next move in the operation might be.

"It was done in a deliberate way. When we had the opportunity, we took it and moved forward into the middle of the city."

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