U.S. ground forces in central Iraq are gathering fresh combat power, probing defenses and allowing time for allied airpower to weaken Iraq's Republican Guard around Baghdad before launching a multi-pronged attack on the capital, officials said Wednesday.

The speed of the initial U.S. ground attack into Iraq from Kuwait last week led many to assume Baghdad would be assaulted soon, but now that appears to be many days away.

Severe sandstorms are affecting the timetable. Apache helicopters that made an initial round of strikes against armor of the Medina division of the Republican Guard on Monday have been grounded since. More Apaches are being brought to the area.

Another snag has been the persistence of fighting around An Najaf, which compelled elements of the 3rd Infantry Division to encircle the city about 90 miles south of Baghdad instead of driving northward.

The United States also opened a northern front Wednesday by dropping 1,000 paratroopers of the Army's 173rd Airborne Brigade into an unspecified location in Kurdish-controlled territory in northern Iraq. Their tanks, other vehicles and supplies will be airlifted in behind them.

A key question was whether the Republican Guard troops -- the best trained and equipped of Saddam Hussein's military forces -- would make the first move by coming out of their dug-in positions on the outskirts of Baghdad, either to attack or to pull back into the urban center.

Reports from the battlefield Wednesday indicated a portion of the Al Nida armored division of the Republican Guard was driving south toward U.S. forces. Others said hundreds of suspected paramilitary forces in civilian vehicles were on the move in roughly the same direction.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said "a few" Iraqis vehicles were heading south from Baghdad toward Karbala, and U.S. warplanes were attacking them. He spoke to reporters Wednesday evening after briefing Senate members on progress in the war. Myers said the vehicles did not appear to be tanks or other armored vehicles.

"They're being engaged as we find them," Myers said.

If there is to be a battle for Baghdad -- and U.S. war planners hope they can topple Saddam's government without one -- it appears the spearheading units, the 1st Marine Division and the Army's 3rd Infantry Division and 101st Airborne Division, are in no hurry to begin.

First they want U.S. and British air power to pound the Republican Guard troops protecting the capital. During the 1991 Gulf War, the Republican Guard was hit with airstrikes until U.S. officials believed its firepower had been reduced by 50 percent. It was only then that the main ground war began.

Loren Thompson, a defense expert at the private Lexington Institute, said air power would need to make up for the relatively light firepower on the ground.

Allied warplanes flew more than 1,500 missions over Iraq Wednesday, including 600 strike stories, a defense official said. The main targets were the Medina and Hammurabi divisions of the Republican Guard and Iraqi leadership targets in Baghdad, the official said.

Iraq's regular army units have not put up much of a fight, although irregular forces like the Fedayeen Saddam -- shock troops not under the military's command -- have launched guerrilla-style raids.

Maj. Gen. Victor Renuart, director of operations for Central Command, which is running the war, told reporters that an important aspect of the original war plan was to close quickly on the Republican Guard divisions "because they are so key to Saddam's success."

But as for when U.S. forces would open a full-scale attack, Renuart said that would be a judgment call for Gen. Tommy Franks, the top commander, in close consultation with his battlefield leaders.

The 3rd Infantry, with more than 200 tanks and other armored vehicles, has been approaching Baghdad from the south, on the west side of the Euphrates River. The 1st Marine Division is driving on a parallel route on the east side of the Euphrates, and at some point the 101st Airborne division may leapfrog to the west or north of the capital, defense officials said.

The 101st Airborne -- the air assault unit known as the Screaming Eagles -- has moved its 3rd Brigade within striking of Baghdad, according to Col. Michael Linnington, the brigade commander.

"We are one tank of fuel from Baghdad," he told an Associated Press reporter traveling with him Wednesday. His and other airborne units were paralyzed by persistent storms.

"The 101st is grounded and we're not doing what we do best, which is air assault operations and attacks," he said. "Once we get the weather break we will restore the offensive to the north."

He said the division's 1st Brigade was expected to join his unit by Thursday and the 2nd Brigade soon afterward. That would bring the division up to its full strength of 20,000 troops.

"When the Screaming Eagles strike, everybody will know," he said.

U.S. special operations forces, meanwhile, parachuted into a desert landing strip in western Iraq as part of missions to keep Iraq from firing missiles at Israel, a senior defense official said. Several hundred U.S. special forces are in northern Iraq, the official said, declining to elaborate on their mission.