WITH U.S. TROOPS, SAUDI ARABIA, JAN. 29 -- Stepped-up bombing of Iraq's elite Republican Guard has "severely degraded" its resupply and reinforcement efforts, but battle damage to fortifications and personnel remains difficult to assess, senior military officials said today.

After 13 days of air raids against the Republican Guard and Iraqi positions in southern Kuwait and Iraq, the officials said allied warplanes were able to travel at will in Iraqi airspace, encountering a weak and dwindling response from antiaircraft artillery and missiles.

A U.S. Navy analyst said carrier planes had bombed the Republican Guard almost unopposed on Sunday. British air force Group Capt. Niall Irving proclaimed "virtual air supremacy" for the allies, saying "we're basically able to operate with impunity wherever we wish."

Still, Col. Manfred Rietsch, U.S. Marines air group commander, noted that the Iraqi guardsmen were "well dug in" behind layers of bunkers, minefields, trenches and revetments, making it hard for analysts to determine what real damage had been done to tanks, artillery and troops. "A ground campaign will be the only way to find out," Rietsch said.

"Softening up" Iraq's fortified positions in occupied Kuwait and southern Iraq is considered crucial to the success of any ground war in the Arabian Desert.

Military planners first delayed heavy bombing of Iraqi troop concentrations in favor of attacks against "hard" military installations and infrastructure. Last week, attacks against the troops were diminished by bad weather.

But Brig. Gen. Pat Stevens IV, head of logistics at U.S. Forces Central Command here, told reporters at a Riyadh briefing that scattered clouds "over a few target areas" Monday and today had "a minimal effect on our operations" and that "the round-the-clock air campaign continues."

Stevens said allied forces flew more than 2,600 sorties in the previous 24 hours, focusing in part on interdiction raids on priority Iraqi command and control installations, airfields, mobile Scud missile launchers and communications.

But Stevens also said more strikes were being launched against the Republican Guard and on troops entrenched on Kuwait's border with Saudi Arabia. "Yes, we're doing some damage," he said of air strikes against the Republican Guard. "I can't quantify that for you, but we're seeing effects . . . on their morale and . . . capabilities."

"I can tell you that at least the ability of the Iraqis to reinforce that part of their armed forces {in Kuwait} has been severely degraded by our attacks on . . . main supply routes," Stevens added.

Lawrence said six carrier-based fighter-bombers struck the Republican Guard on Sunday without encountering significant ground fire, surface-to-air missiles or Iraqi jets. "That pretty much surprised us. The missiles . . . are way down, and the antiaircraft artillery is down from what we experienced the first week," Lawrence told combat-pool reporters Monday. "It looks like the poor army guys have been left undefended by the air force and the antiaircraft artillery."

British Group Capt. Irving also noted that antiaircraft fire over the Republican Guard had "progressively grown thinner" as the air offensive unfolded. Air Force debriefers of A-10 "Warthog" ground-attack pilots reported almost no resistance over their targets in Iraq today.

Lawrence said Iraqi antiaircraft artillerymen seemed "low on ammunition and low on will to fight." He said Iraqi surface-to-air missiles had also been ineffective, suggesting that allied countermeasures had caused enemy gunners to shut down their fire-control radar.

"About all the SAMs they have fired that we have seen so far are unguided," Lawrence said. "Because they are afraid of our {air-to-surface} HARM missiles, they blink {radar} on and off, or just leave them off." The HARM missiles home in on radar beams.

"I think the army guys have been left to bury themselves," Lawrence said. "All those guys in the Republican Guard are getting bombed at will. They are just dug in."

Despite their optimism, military analysts have cautioned that assessments of air-raid damage to troop concentrations and entrenched armor are difficult to make. "They're waiting; they've hidden their tanks," said Marine Col. Rietsch. "When we'll be able to destroy them in large numbers is when they bring them out and move them."

A few days ago, Rietsch said, his pilots "by luck" came upon a truck convoy whose drivers had stopped for Moslem prayers, and they attacked with cluster bombs. "That was very much a fun mission because you could see the parts of the trucks flying," Rietsch said. But he and other Marine pilots said it is rare to find an open target near Iraqi trenches.

Brig. Gen. Stevens said the attacks nevertheless were having a severe effect on Iraqi supply lines, both to the Republican Guard and to troops on the Saudi border. While these troops had "stockpiles there as any prudent force would do," the allied raids "are causing him to use some of those supplies he built up."

"That's why it's so important for us to go after their current main supply routes, these avenues to keep that resupply coming," Stevens said. "We're having an effect on that. We're also targeting those supplies that he's brought forward. We'll continue to do that. There will be a gradual erosion of his capability to wage war."