The search by the United States and its allies for Scud missiles in Iraq is complicated by the compactness, simplicity and mobility of the weapons, U.S. military officials said yesterday.

The Soviet-made Scuds are roughly the length of a telephone pole and only three feet in diameter, and can be fired from the back of any large truck mounted with a launcher. Scud batteries emit few telltale electronic signals and, between launches, can be hidden beneath bridges or in tunnels, trenches or warehouses, military analysts said.

Spy satellites have had some difficulty distinguishing them from other objects, officials said.

Various U.S. reconnaissance satellites, electronic intelligence-gathering sensors and at least 10 types of military aircraft were involved in the hunt for the elusive missiles yesterday, officials said.

In some cases, the planes and intelligence-gathering devices were diverted from their planned missions to assist in what has now become a top U.S. priority in the war.

U.S. officials ordered the massive effort after Iraq fired 11 Scuds at Israel during the first three days of combat in a so-far unsuccessful effort to draw Israeli armed forces into battle.

"We're putting a lot of assets against the problem and doing everything we can to find and destroy the mobile Scud launchers," Defense Secretary Richard B. Cheney said after meeting with President Bush yesterday.

Marine Maj. Gen. Robert Johnston, the chief of staff for the U.S. Central Command, estimated yesterday that Iraq had roughly 30 fixed missile launchers and more than 20 truck-mounted launchers.

U.S. intelligence agencies estimated before the war that Iraq had a stockpile of 300 to 1,000 missiles capable of being fired from these launchers, although military officials believe some Scud storage sites have been destroyed since the war began last week.

Military experts said that fueling and preparing a Scud missile for launch can take hours, but would still be difficult to detect if rudimentary efforts are made to keep the missile and launcher hidden.

The fuel is a volatile liquid and the safety precautions that should be taken would be readily observable, and U.S. officials say the Iraqis sometimes do take such precautions.

"The best way to find them may be with armed {aerial} reconnaissance missions," in which many planes slowly search a suspect region, a former U.S. military officer said on condition of anonymity. He said U.S. and allied forces were able to destroy several Scud missiles Friday because an Air Force A-10 pilot sighted one from his cockpit.

Mobile Scud units are mostly organized in brigades of six launch vehicles that use one set of command and support vehicles, including equipment to test the missiles and a crane to place them on the truck launchers.

Because they usually travel in groups, when one is sighted, others are usually nearby, which is why six launchers were discovered near Basra in a brief period Friday.

The brigades must be located near roads or well-established tracks in the desert. "The launch vehicles . . . don't go like dune buggies," said David Isby, a military analyst in Washington who specializes in Soviet-made weaponry.

"The great fear is that they may have done something like what the Syrians did in the 1973 war against Israel, when they pre-surveyed their launch sites and kept {surface-to-surface} missiles hidden in tunnels. They would run them out, fire them and run back," Isby said.

U.S. officials said the missile is also vulnerable to detection during final stages of launch preparations, when Iraqi forces release balloons to check for wind flow that could divert the missile from its target.

The Iraqis monitor the movement of the balloons using small radars that Western military officials have code-named End Tray and Bread Bin. These radars transmit telltale signals.

U.S. experts say that finding such radars can be complicated, however, by the Iraqi practice of deploying extra radars that send identical signals to confuse potential attackers.

Iraqi Scud commanders practiced this method during the 1980-88 war with Iran, after Iranian forces learned to shell Scud missiles by homing in on the radar signal.

One official said the Iraqis might also try to skirt the need for weather balloons by firing a missile and calibrating later missile firings by observing its trajectory.

Officials said that the hunt involves U.S. Airborne Warning and Control System aircraft, EF-111 and EC-130 electronic warfare planes, and high-flying U.S. TR-1 spy planes equipped with special radar that can peer a great distance. Ground-attack planes such as F-111 bombers and F-15, F-16 and A-10 aircraft have also been diverted to the effort.

Staff writers Evelyn Richards and James Rupert contributed to this report.

THE OBJECT OF THE SEARCH

1. Scud missiles and launchers can be hidden in tunnels, trenches or buildings, then transported to an open-air launch site. It takes a brigade team three to six hours of on-site preparation before a missile can be launched.

2. Normal brigades consist of six mobile launch trucks. The vehicles cannot cross rough terrain. They must be located near roads or clear, well-established desert track. Accompanying vehicles include a crane to mount the missiles, and command and support vans.

3. The team sends up a balloon to determine high-altitude winds that could affect the missile's course. The balloon is tracked by radar equipment, which enemy forces may try to pinpoint. The launch team can activate additional radar to mislead the enemy, a tactic used during the Iran-Iraq war.

4. With information gleaned from the balloon ascent, technicians send final targeting information to the launch vehicles. The vehicles may be linked by radio, which can be intercepted, or the launch site may be equipped with previously installed phone lines that cannot be monitored.

5. The missiles are launched.

TOOLS OF THE SEARCH

The military uses a variety of aircraft to perform the following tasks, including AWACS, F-4G Wild Weasels, EF-111s, EC-130Hs, RC-135s and TR-1s.

Radar: Overhead or ground-based radar can be used to search for Scud launchers in enemy territory. But it is difficult to distinguish the missile launchers from ordinary trucks, tanks and other large metal objects.

Electronics intelligence: Computerized monitors can be used to "eavesdrop" on radar signals from balloons at the launch site, and eavesdrop on other communications.

Satellite Photography: U.S. satellites orbiting hundreds of miles above the Earth can photograph objects as small as six inches wide, then send the images to analysts for interpretation within minutes.

Infrared: Systems that look for heat could pinpoint Scud sites at time of launch. Satellite-based infrared systems can detect the plume of heat emitted by a launched missile, but analysis is needed to distinguish between this and other heat sources.