Sandstorms slowed U.S. and British forces to a crawl and thwarted air missions Tuesday as U.S.-led forces edged closer to the Iraqi capital. Baghdad residents, hunkered down for an eventual battle, woke to howling winds and the distant crash of artillery.

Combat missions from two aircraft carriers were called back because of bad weather. At least a dozen planes returned without reaching Iraq. Two Army divisions were virtually stalled in a vicious sandstorm that reduced visibility to a few feet.

Thousands of Marines were trekking toward Baghdad, taking back roads to avoid civilians, but they traveled only about 20 miles in five hours with visibility at about 10 feet. A traffic jam of military and supply vehicles was buffeted by heavy winds and blowing sand.

The Army's 3rd Infantry Division advanced to within 50 miles of Baghdad early Tuesday and pounded military installations with howitzers and rockets. U.S. warplanes and helicopters had come under heavy fire Monday during their first attacks on Saddam Hussein's elite ground units. In southern Iraq, tenacious resistance kept the coalition from securing key cities.

The helicopter assault on Saddam's elite Republican Guard units Monday was the first known engagement in central Iraq, and many U.S. aircraft were hit by Iraqi ground fire. About 10 Iraqi tanks were destroyed in the attack. One U.S. helicopter went down, and the Pentagon said two pilots had been taken prisoner - Chief Warrant Officer Ronald D. Young Jr., 26, of Lithia Springs, Ga., and Chief Warrant Officer David S. Williams, 30, of Orlando, Fla.

U.S. officials said they believe Iraq is more likely to use chemical or biological weapons against coalition troops the closer they get to Baghdad. The Republican Guard controls the bulk of Iraq's chemical weaponry, most of which can be fired from artillery guns or short-range rocket launchers, according to U.S. officials.

A day after Saddam urged his people to resist the invaders in a rousing televised speech, Iraqis set up mortar positions and piled sandbags around government buildings and other strategic locations on the southern outskirts of the capital.

A pattern of deadly ambushes and ruse attacks by Iraqi militiamen in civilian clothes prevented coalition forces from securing the southern cities of Basra and An Nasiriyah and sporadic fighting thwarted efforts to extinguish burning oil fields. British officials decided Tuesday to target pro-Saddam militiamen in Basra; previously coalition commanders had said they would avoid urban combat in Iraq's second-largest city.

''These things are never easy,'' British Prime Minister Tony Blair said Monday. ''There will be some difficult times ahead, but (the war) is going to plan despite the tragedies.''

Heavy fighting continued in An Nasiriyah, considered a strategic prize because of its bridges across the Euphrates. Navy pilots pounded Iraqi artillery and ammunition posts about 45 miles northwest of Basra overnight into Tuesday morning, U.S. officials said. Two British soldiers were killed at Az Zubayr, an Iraqi navy port not far from Basra.

Air Marshal Brian Burridge, commander of British forces in the Gulf, said a Baath party headquarters in Az Zubayr was targeted late Monday by the First Battalion Black Watch, whose members are mostly from Scotland.

''We went to their headquarters and engaged in contact with them … and made it quite clear to them: We, the British forces, are up for this, and you are going to have a very hard time,'' Burridge told British Broadcasting Corp. radio Tuesday.

At least one Baath party official was captured and 20 were killed in the operation, which was over within hours, British spokesman Col. Chris Vernon told the BBC.

Troops advancing toward Baghdad said they were prepared for the fight ahead, despite news of fallen comrades. At least 20 U.S. troops have been killed and 14 captured or missing since the operation began.

''I think the deaths of Americans gives us more incentive to fight,'' said Lance Cpl. Chad Borgmann, 23, of Sidney, Neb., with the 15th Marine Expeditionary Force. ''Freeing Iraq is all fine and dandy … but this gives us a personal motivation to fight.''

Loud explosions were heard Tuesday near the northern oil center of Kirkuk, but people in the Kurdish-held city of Chamchamal, about 30 miles to the west, did not see any planes in the sky. Coalition warplanes bombed Iraqi military barracks in the area on Monday.