RIYADH, SAUDI ARABIA, FEB. 6 -- As U.S. and allied warplanes step up the most difficult phase of the air campaign against Iraqi forces, the military is struggling to assess the impact of round-the-clock raids on widely dispersed troops and supply lines on the battlefield.

"It's much more difficult to say you've taken out 30 tanks in the desert than to say you took out a chemical plant or a bridge," said a senior U.S. officer involved in the operation. "And it's much easier if you close with the enemy and can walk over and see the tanks you destroyed. It's much more difficult when you're fighting from a distance as we are now."

A further complication, according to senior officers, is that the Iraqis have honed their skills at deception, including use of decoys.

The most skillful at deception, other officials note, are the elite Republican Guard units, which are the highest priority target for the allied bombing campaign. Like much of the rest of the Iraqi military, the Republican Guard uses numerous devices to carry out those efforts.

Sheets of aluminum placed under camouflage netting can deceive radars into believing an armored vehicle sits beneath the cover, leading pilots to waste valuable bombs on a fake target. In other instances, when allied bombs and missiles do not hit a real tank, Iraqi troops set off large smoke bombs to make the pilots believe they scored a hit, discouraging them from returning to the area until satellite photographs reveal the tanks are still operational.

Allied intelligence officers are sifting through thousands of satellite photographs, radio interceptions, pilot reports and other data to assess the damage to Iraqi forces, according to officers.

"Intelligence is a mosaic made up of thousands and thousands of bits of information," said a senior officer. "It's more of an art than a science when you're making an estimate of what the enemy capability is."

Military authorities who spend their days and nights in the war room here say they know much more about bomb damage than they are willing to discuss publicly. Field commanders have said in recent days that they are provided daily reports with extraordinarily detailed bomb damage assessments, according to pool reports.

Army commanders said the latest assessments indicate that only 10 percent of the Iraqi highway transportation network remains intact but that Iraq's power generation system is now operating at about 75 percent of its total capacity.

Officers also said they receive frequent updates on the number of Iraqi tanks and armored personnel carriers that have been destroyed. Senior military officials said today that about 70 percent of the Iraqi military's communications network has been destroyed or damaged.

Capt. Terry Featherston, an A-10 attack jet pilot who flies frequent missions against battlefield targets, told reporters, "Every night we search, and every night it is more difficult" to find fresh targets because the landscape of Iraq and Kuwait has become so littered with destroyed armored vehicles and other damaged war equipment.

Featherston, 29, of Joliet, Ill., said that in the first week of the war he could drop his load of Rockeye anti-armor cluster bombs, infra-red Maverick missiles and 500-pound iron bombs in a single pass over the battlefield. Now, three weeks later, the pilot said he sometimes must make six or seven passes to find enough good targets.

The A-10 pilot and dozens of his colleagues flying bombers, fighters and attack planes have provided some of the most vivid anecdotal details of damage to Iraqi forces. Still, the military has steadfastly refused to assemble a more complete picture for public dissemination.

While officers say they are hesitant to release details of bomb damage that would provide valuable intelligence to the Iraqis, the U.S. war operation here is dogged by persistent ghosts of the Vietnam War, when commanders paraded optimistic statistics and body counts before the public even when American forces were losing battles.

"We will continue to be deliberately conservative in what we tell you and the American people," senior allied commander Gen. H. Norman Schwarzkopf told a small group of journalists this week. "We don't want a credibility gap to generate."

Still, senior officers concede that their intelligence assessments involving the Republican Guard are among the sketchiest of the entire operation.

"The Republican Guard is harder to talk about because it's further back," said Schwarzkopf. "The deeper they are, the less intelligence you're going to have on them."

The Republican Guard also is the force that allied officials are most reluctant to discuss. British Royal Air Force spokesman Group Capt. Niall Irving told reporters here that evidence of significant devastation within those forces will signal preparations for an allied ground attack. A senior French officer said today that bombing has destroyed 30 percent of the Republican Guard's capabilities, a figure that American authorities would neither deny nor confirm.

The Republican Guard forces -- 150,000 strong before the war began -- are being bombarded by "packages" of three B-52 bombers that drop cluster bombs and other ordnance on their positions every three to four hours, officials said.

"We've been hitting them harder than we're hitting anyone else," said Schwarzkopf.

British military spokesman Irving said many Iraqi troops are being forced to communicate outside of secure networks on regular VHF radio channels, which are far easier for allied intelligence gatherers to intercept. Other authorities noted that such open communication could allow allied special forces teams to infiltrate Iraq's airwaves with false battle orders designed to confuse Iraqi troops.

Senior officers also said they are gleaning some intelligence information from the more than 800 Iraqi prisoners of war now in the custody of American and Saudi forces. U.S. officers involved in the interrogation of the prisoners told pool reporters that the men said their units were suffering nightly casualties as a result of the allied air raids.

This week the allied forces have begun shifting more of their missions against other Iraqi ground forces and their support networks. About one-fourth of the sorties flown in the last two days have been directed at front-line and second-echelon ground forces in Kuwait and southern Iraq, officers said.

"That's going to do nothing but increase," said a senior U.S. officer.

The targets also have become much more fluid. Rather than stationary buildings and military factories, pilots flying nightly missions say they are "trolling" the highways of Kuwait, looking for "movers."

"It's very similar to if you're fishing with your line out and hoping to find something," said Lt. Col. William J. Horne, commander of the Marine All-Weather Attack Squadron 224 based at Cherry Point, N.C. "We just patrol, hoping to find enemy movers at night and attack them."

The pilots said, however, that the Iraqis are changing their transportaton patterns to avoid detection.

Marine Maj. Thomas J. McElrath of Pompton Lakes, N.J., flies A-6 Intruder attack planes. Several days ago, he said, he flew over a convoy with 24 vehicles on one side of the road and 27 on the other side of a road between Kuwait City and the Saudi border. "It looked like a centipede," McElrath told pool reporters. "Last night we were patrolling and there was nothing on the roads."

But senior U.S. officers here say they frequently are skeptical of pilot reports and use them as only a fraction of their overall battle damage assessments. "We get the pilot reports in, then divide them by two and sometimes again by four," said a senior American officer.

While pilots involved in attacking invading Iraqi forces from Kuwait last week reported that they hit 44 tanks, the military later estimated that about half that number were destroyed.

Staff writer Guy Gugliotta in Dhahran contributed to this report.