(CNN) -- American tanks and armored vehicles rumbled into Baghdad Monday, storming one of Iraqi President Saddam Hussein's main presidential palaces and destroying symbols of his regime.

South of the Iraqi capital, U.S. troops found drums of what may be chemical weapons materials at an agricultural complex near Karbala, military officials said Monday.

Gen. Benjamin Freakly of the 101st Airborne Division said tests performed Monday indicate the presence of nerve and blister agents, but he added, the test sometimes shows false positives. More samples are being tested.

Meanwhile, some 25 miles east of Mosul -- the largest Iraqi city remaining under regime control -- coalition forces Monday were approaching the main highway connecting Mosul to Kirkuk, hoping to cut off those cities from each other.

The move comes as coalition forces consolidate positions in an effort to choke off and surround all major Iraqi cities, including those in the north.

According to defectors, there are no Republican Guard troops operating in northern Iraq, but there has been very active Iraqi artillery resistance during the coalition bombings, which have taken place almost every day for the past two weeks.

In Baghdad, Iraqis offered little resistance, but later an Iraqi missile slammed into the tactical operations center for the U.S. Army's 2nd Brigade of the 3rd Infantry Division.

At least four people, including two soldiers and two journalists, were killed and 15 were wounded in the strike, military sources said, while two Marines died in a separate firefight east of Baghdad.

In addition, at least nine Iraqi civilians died and 13 others were wounded when an apartment block in Baghdad's Mansour neighborhood was hit, according to CNN sources who went to the scene. A large crater was visible, and a restaurant and an apartment building were destroyed.

Iraqis told the Lebanese Broadcasting Corp. they believe coalition air raids caused the damage. U.S. Central Command in Qatar had no immediate comment.

"We did not originally leave [Baghdad] because we were confident that we were not going to be bombed, but the Americans who claimed they came here under the slogan, 'Rescue the Iraqis,' did indeed bomb us," a resident told the Lebanese network.

Fighting also raged in southern Iraq, where British forces increased their control over Basra, as well as northern Iraq, where coalition forces captured a key ridge between Mosul and Kirkuk.

U.S. officials characterized earlier incursions into Baghdad as raids and reconnaissance missions, but Army sources said that the 2nd Brigade units will not withdraw from the city after moving in Monday.

"The commanders on the ground will make the decisions on what parts of Baghdad they wish to retain control of," Brig. Gen. Vincent Brooks said at a Central Command briefing.

Brooks noted that coalition forces destroyed numerous Iraqi tanks, armored personnel carriers and infantry units in the northwest part of the capital.

U.S. tanks rolled through the city's parade field, adjacent to the Al-Rashid Hotel in the heart of the city, Atlanta Journal-Constitution reporter Ron Martz told CNN. American forces blew up a 40-foot statue of Saddam in Baghdad's Zawra Park, Martz added, and struck several other symbolic and functional regime targets.

Several fires burned near one of Saddam's palaces on the northwest bank of the Tigris River as people ran away from U.S. tanks, according to video shown on the Lebanese Broadcasting Corp.

"It can't be anything but alarming to see a [coalition] brigade commander standing in the compound of a presidential palace in Baghdad," a Pentagon official said.

"This sends a powerful message to the remnants of the regime that we can go where we want when we want."

An Army officer said some U.S. soldiers tried to raise an American flag, incensing some Baghdad residents, according to CNN's Walter Rodgers, who is embedded with the 3rd Squadron, 7th Cavalry of the 3rd Infantry Division.

Standing in the smoke-filled streets of Baghdad on Monday, Iraqi Information Minister Mohammed Saeed al-Sahaf refused to acknowledge the U.S. raids.

"The soldiers of Saddam Hussein have given them a lesson they will never forget," al-Sahaf said.