U.S. soldiers rolled into a base in the Shiite holy city of Najaf (search) on Monday to replace withdrawing Spanish troops and put pressure on a radical anti-American Shiite militia that controls parts of the city.

The base is about three miles from the Shiite holy shrines at Najaf's heart, which the U.S. military has vowed to steer clear of to avoid outraging Iraq's majority Shiite Muslims (search).

The move deploys U.S. troops within the Najaf urban area for the first time since a large force massed outside the city earlier this month to put down the Al-Mahdi Army (search) militia of radical cleric Muqtada al-Sadr (search).

About 200 troops and Military Police rolled into the base Monday morning, in part to prevent the site from falling into the militia's hands after Spanish troops withdraw. Col. Paul White said the Spanish would be leaving in the next few days.

Overnight, al-Sadr's forces shelled the base with 21 mortars, wounding at least one Salvadoran soldier, said Col. Paul White, commander of the U.S. 2nd Battalion, 37th Armored Regiment, which moved into the base.

"We are going in to allow the Spanish troops to leave safely and so that the compound is not left empty," said White, commander of the 2nd Battalion, 37th Armored Regiment, which moved into the city. "We don't want al-Sadr's militia to take it over. It is not an offensive operation."

Spain, with some 1,300 troops, leads the Plus Ultra brigade in Iraq, a command that also includes forces from El Salvador, Nicaragua and the Dominican Republic.

Last week, newly elected Spanish Prime Minister Jose Luis Rodriguez Zapatero, who opposes the U.S.-led war in Iraq, ordered Spanish troops home as soon as possible. Zapatero's Socialists won the elections three days after the March 11 terrorist bombings of a Madrid commuter train, which killed 191 people and wounded more than 2,000.

The limited move into the base also gives U.S. forces a foothold in Najaf from which to pressure al-Sadr, who is holed up in the center of the city near the shrines, where his militiamen largely control the streets.

The base — actually two adjacent bases called Baker and Golf, one with Salvadoran troops, the other with Spaniards — lie in the modern part of Najaf, an urban extension that melds with the neighboring city of Kufa.

U.S. officials said they would start "economic, military and psychological" operations to pressure al-Sadr. Brig. Gen. Mark Hertling, commander of the 1st Armored Division, said U.S. troops would build up Iraqi security forces in Najaf.

"We're going to drive this guy into the dirt," Hertling said of al-Sadr on Sunday. "Either he tells his militia to put down their arms, form a political party and fight with ideas not guns — or he's going to find a lot of them killed," he said.

Shiite leaders have warned of a possible explosion of anger if U.S. forces enter the holy sites.

Coalition spokesman Dan Senor said Sunday that weapons were being stockpiled in mosques, shrines and schools in Najaf and demanded an end to the practice. He said that Amb. L. Paul Bremer, the top U.S. official in Iraq, met with Iraqi journalists Sunday and warned that the practice threatened stability in the area.

Al-Sadr's militia launched a bloody uprising on April 4, and took control of police stations and government buildings in several cities in largely Shiite southern Iraq. Al-Sadr is based in Najaf, home to the Imam Ali Shrine, one of the holiest sites in Shiite Islam.

Al-Sadr's gunmen left the stations and buildings a week after the uprising, but his militiamen can still be seen in the streets of Najaf and the nearby city of Kufa, carrying assault rifles and rocket-propelled grenade launchers.

U.S. forces have been surrounding Najaf since shortly after the uprising but commanders have been reticent to launch an attack in which they could end up fighting militiamen hiding out in shrines and mosques that are considered sacred by Shiites.