American armored combat troops moved through "the heart of Baghdad" on Saturday from the south and coalition troops also took several objectives surrounding the capital in the north and northwest, U.S. military officials said.

In addition to a bold daytime raid by U.S. armored forces into Baghdad from the south, coalition forces were surrounding the Iraqi capital to prevent reinforcements from entering, said Maj. Gen. Gene Renuart, operations director at U.S. Central Command.

"[It's] essentially isolation," he said. "We'll continue to operate with our forces around the city to prevent forces from coming into the city and challenging us."

A senior U.S. military official later said coalition forces also have seized several objectives at multiple points surrounding Baghdad, including areas north and northwest of the city. The official declined to identify sites taken.

"We're either in control of these objectives or in the process of taking them," said the official, who spoke on condition of anonymity.

Previously, U.S. Central Command had only acknowledged having troops in the southern part of the city and special operations forces along the roads leading from Baghdad to Tikrit, home town of President Saddam Hussein.

Navy Capt. Frank Thorp, a coalition spokesman, said U.S. forces had entered "the heart of Baghdad." He said there was intermittent fighting.

"Resistance has been sporadic, but when it's happened, there has been some fierce fighting," he said. "I would not want to down play the level of resistance on the ground."

Fox News reported that a column of 26 American tanks and 10 Bradley fighting vehicles from the 3rd Infantry, 2nd Brigade entered Baghdad early Saturday and engaged Republican Guard troops and paramilitary forces. Footage broadcast by Fox showed burning vehicles near a deserted highway through a desolate stretch of land.

Fox correspondent Greg Kelly said one U.S. tank was destroyed.

Thorp declined to be specific on how many troops moved through the city of 5 million, but described the numbers as "substantial."

"We have reports of engaging the special Republican Guard in a limited basis," in Baghdad, Thorp said.

However, witnesses in the capital said they saw no evidence of an incursion in the center of Baghdad. Thorp refused to specify what he meant by the city's "heart."

He said Baghdad was a big city, and that "if you move through the heart of New York City, there are going to be millions of people who don't see you."

Thorp stressed that "American armored combat formations have moved through the heart of Baghdad, defeating the Iraqi troops we have encountered.

"We have substantial forces now moving into the city," he said.

Later Saturday, Renuart described the incursion into Baghdad as a movement of tanks and other armor through the southwest quadrant of the capital that ended at the international airport west of the city.

"It was I think a clear statement of the ability of the coalition forces to move into Baghdad at a time and place of their choosing," he said.

Renuart said the objective was not "to hold any territory in Baghdad. This was an opportunity that the ground force commander saw to move troops through a major area of Baghdad, and jumped on it."

Earlier, Iraqi Information Minister Mohammed Saeed al-Sahhaf had disputed reports that coalition forces controlled the international airport, saying Iraqi forces had driven allied troops from the airport. The coalition says the airport is securely in the hands of American forces.

Thorp also said elements of the 1st Marines Expeditionary Force had penetrated the Al Nida division of the Republican Guard on the southeastern outskirts of Baghdad.

Brig. Gen. Vincent Brooks told reporters Friday that special operations forces were "maintaining effective control" of the roads between Baghdad and Tikrit, hometown of Iraqi President Saddam Hussein.

That puts coalition forces along the southern arc of Baghdad as well as a northern entry point, although Thorp declined to say where else they might be.

The move into the city came after U.S. Army tanks rumbled into Baghdad early Saturday for the first time while columns of armored vehicles began encircling the city.

Thousands of Iraqis fled the city, fearing urban warfare.

The Iraqi capital was breached by a reconnaissance force of the 3rd Infantry Division. It came as Army tanks and infantry fought off attacks at the international airport.