The Associated Press

Coalition troops pressed toward Baghdad with new wariness on Monday, as militiamen loyal to Saddam Hussein proved they were not a beaten force and sandstorms snarled the advance. Iraq claimed to have shot down two U.S. helicopters and taken two pilots prisoner, a day after more than 20 Americans were killed or captured.

Facing a pattern of deadly ambushes and ruses, and with many of Saddam's supporters discarding their uniforms in favor of civilians clothes, coalition forces responded with tough new tactics in the south. U.S. officials also confirmed their forces have arrived in northern Iraq, and have been operating there for about 24 hours, though few details were released.

Saddam, in an appearance that seemed calculated to show he remained at the helm, sought to rally his people Monday with a televised speech. Iraqi TV later showed images of what appeared to be a downed U.S. Apache attack helicopter sitting largely undamaged in a grassy field. Information Minister Mohammed Saeed al-Sahhaf claimed peasants had shot down two Apaches, and two pilots were in custody.

U.S. Gen. Tommy Franks later confirmed that one helicopter and two pilots were missing in action.

A brutal sandstorm with howling winds stalled U.S. troops about 50 miles south of Baghdad, near Karbala, a city holy to Iraq's majority Shiite Muslims. As the 7th Infantry Regiment pressed north, Iraqi militiamen shot mortars at a supply convoy of Humvees and cargo trucks. There were no casualties. Commanders requested air support, but the Iraqis were able to flee easily in small, civilian trucks.

Armed members of Saddam's Baath party militia have increasingly been attacking coalition supply lines in this way.

With tension about such attacks mounting, Marines operating in the south took a tough tact with Iraqis they encountered Monday, forcing men from vehicles, questioning them and then slashing their tires to make sure they wouldn't harass other convoys.

In the Iraqi navy port of Az Zubayr, which the coalition claimed Sunday, a Marine patrol reported being fired upon from a stand of trees; Marines responded with tanks and artillery fire. It was not immediately clear who fired on the patrol, and there were no casualties.

Coalition officials rejected suggestions that continued Iraqi resistance or casualties had knocked war plans off balance.

"I think that within three days of real military operations beginning, the idea that somehow people are losing confidence or heart is nonsense," Defense Secretary Geoff Hoon said. "This is a difficult, demanding, complex, sophisticated military operation. It is not going to be over in a matter of days."