The Associated Press

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.

The Army met sporadic resistance on its journey north. A report from the 3rd Infantry Division's headquarters estimated 500 Iraqis were killed during a two-day sweep past the holy Shiite city of Najaf, said Command Sgt. Maj. Kenneth Preston of the Army's V Corps.

At least 20 U.S. troops have been killed and 14 captured or missing since the operation began.

Providing an overview of the state of the military campaign, British Prime Minister Tony Blair said the coalition had secured a key southern port despite tenacious resistance, and had much of the western desert in hand; air attacks were launched on targets in the northern oil centers of Mosul and Kirkuk, as well as Saddam Hussein's hometown of Tikrit, and troops were making advances in eastern Iraq to help defend two main bridges over the Euphrates.

Blair stressed that the final miles on the road to Baghdad would be the most challenging, as U.S. Army troops face the Medina division of Saddam's Republican Guard.

"This will plainly be a crucial moment," he said.

Combat missions from two aircraft carriers were called back because of bad weather and two Army divisions were virtually stalled in a vicious sandstorm that reduced visibility to a few feet. Thousands of Marines trekking north toward Baghdad traveled only about 20 miles in five hours, buffeted by heavy winds and blowing sand. While Iraq often sees sandstorms in the spring, meteorologists said this one was exceptional.

The Army's 3rd Infantry Division advanced to within 60 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's elite Republican Guard units.

In Monday's helicopter assault, the first known engagement in central Iraq, about 10 Iraqi tanks were destroyed. One Apache 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. Military officials later said they had destroyed the helicopter, but did not say how.

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.

Coalition forces destroyed six satellite jamming devices, which Iraq was using to try to thwart American precision guided weapons, Air Force Maj. Gen. Victor Renuart said. He said the devices have had "no effect" on U.S. military operations, said .

President Bush had called Russian President Vladimir Putin to complain about the alleged sales of high-tech equipment, which could pose a threat to American troops. The Kremlin denied the allegations that Moscow had broken U.N. sanctions to sell such weapons.

A pattern of deadly ambushes and ruse attacks by Iraqi militiamen in civilian clothes have hampered the efforts of coalition forces in the south, and sporadic fighting forced firefighters to withdraw from burning oil fields. British officials said Umm Qasr, Iraq's only deep-water port, was secure, though it could take several days before humanitarian aid deliveries begin because the waterway must be swept for mines.

British officials also said they would target pro-Saddam militiamen in the main southern city of Basra; coalition commanders had previously said they wanted to avoid urban combat there.

"Basra is surrounded and cannot be used as an Iraqi base," Blair said in a news conference Tuesday. "But in Basra there are pockets of Saddam's most fiercely loyal security services who are holding out. They are contained but still able to inflict casualties on our troops, and so we are proceeding with caution."

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 Persian 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.

"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.