RIYADH, SAUDI ARABIA, FEB. 16 -- One month into the Persian Gulf War, U.S. and allied forces have started trial runs for a combined assault from land, sea and air to retake Kuwait.

The change in emphasis in allied air and ground movements has become particularly evident in recent days across the vast desert battlefield as Operation Desert Storm moves into a new stage that recognizes the need for increased softening-up assaults on Iraqi defenses around Kuwait prior to the long-awaited ground offensive.

In one apparent trial run early today, a squad of AH-64 Apache helicopters in a combined operation with artillery and multiple rocket launchers conducted a predawn raid against Iraqi positions in Kuwait, the allied command announced. The assault was the war's first known night mission combining Apache gunships with other arms.

The Apache mission destroyed four Iraqi vehicles, two radar vans, two reconnaissance outposts and a bunker, U.S. military sources reported. The helicopters also blasted a group of Iraqi troops with rockets and cannon fire but received no return fire, they said.

Military officials refused to specify where the assault took place, saying only that it was just north of the Saudi border. Such combined-weapons operations, including a large-scale helicopter assault, have been cited as likely tactics for the beginning of an all-out allied ground offensive.

Relentless bombing of Iraqi armored forces during the last three weeks, including the crack Republican Guard, has seriously reduced the combat effectiveness of Iraqi divisions assigned to defend Kuwait along the Saudi border or stationed farther north as reserves along the southern rim of Iraq, U.S. officers said.

Brig. Gen. Richard I. Neal, Desert Storm's deputy chief of operations, estimated Thursday that U.S. and allied bombers have reduced Iraqi armor and artillery in the Kuwait theater of operations by about one-third and are knocking out scores more every day. By the Pentagon's count, allied forces have now destroyed 1,400 tanks, 800 armored personnel carriers and 1,200 artillery pieces.

As a result, senior U.S. military officers said air and ground attacks designed to "frame" the battlefield for a U.S. offensive are taking on increasing importance as U.S. target selectors at allied headquarters in Riyadh plot the day's bombing runs. Without calling off the attacks against strategic targets in central Iraq or armor concentrations along the Iraqi-Kuwaiti border, they added, targeters are directing a growing number of warplanes against front-line troops and their elaborate defenses along the frontier with Saudi Arabia.

In another sign that direct preparations for ground war are increasing, some U.S. planes have begun dropping fuel-air explosives, including 10,000-pound BLU-82 "daisy-cutter" bombs to test their effectiveness in clearing minefields, berms and wire obstacles erected in the path of any advancing allied forces.

A fuel-air weapon disperses a petroleum vapor cloud and then ignites it, causing a huge fireball and shock wave. Neal said these explosives are employed against mines and light equipment such as trucks. Their fireballs also suck away oxygen, which specialists pointed out could lead to asphyxiation of Iraqi troops hiding in bunkers.

For all the allied successes in an air campaign of more than 76,000 sorties so far, infantry officers acknowledged that most of Iraq's 530,000 soldiers remain hunkered down in a network of bunkers protected by rings of berms and minefields. More than 1,200 Iraqis have become prisoners of war or defectors, signaling flagging morale, but Neal and other officers repeatedly have warned that the Iraqi defenders' will to fight remains an open question that may be answered only in battle.

Against this background, the increased tempo of preparations for a ground assault has gone unabated despite Iraq's peace overture Friday. Reconnaissance showed no sign Iraqi troops were preparing to evacuate Kuwait, and radio intercepts overheard no orders going out from Baghdad telling Iraqi commanders the war is nearing an end, U.S. officers said.

Instead, Neal and other allied officers interpreted Saddam's proposals as a feint designed to gain respite from the bombing and force the United States to put off any ground offensive. Despite a swell of speculation that the ground war is imminent or that Saddam's maneuvers could accelerate the timetable, U.S. officials said President Bush has not yet notified the Desert Storm commander, Gen. H. Norman Schwarzkopf, that the time has come.

"We're continuing our campaign plan as we conceived it," Neal said.

"It may be days; it may be weeks off yet," declared the commander of British forces in the Middle East, Air Chief Marshal Patrick Hine, after a meeting with Schwarzkopf.

U.S. military officials from the Pentagon down to Marine colonels in their sand-bunkered tents have emphasized that, if a ground assault is ordered, they will strike at Iraqi troops in Kuwait with a combined-weapons operation seeking to unite allied naval, air and land forces in a single, massive attack.

Iraqi forces gained no experience with such combined operations during the 1980-88 Iran-Iraq war, U.S. military officials pointed out, and have displayed little ability to deal with them so far in the present conflict.

Neal, briefing reporters here, said allied planes flew 700 of their total of 2,600 sorties in the last 24 hours against Iraqi troops and equipment in the Kuwait theater of operations. With about half the sorties for supply runs and refueling operations, that meant more than half the combat strikes hit the defensive rings of Iraqi front-line forces. More than 170 others were directed against Republican Guards farther north and another 150 aimed at Scud missile launchers, he said.

Since the initial high-tech onslaught against strategic command and communications installations in Iraq that opened the devastating air campaign shortly before 3 a.m. on Jan. 17, U.S. and allied warplanes have gradually shifted their emphasis southward in accordance with what U.S. officers describe as a detailed but constantly changing plan for war.

Without abandoning the attacks on command posts in Baghdad and other Iraqi cities, U.S. and allied aircraft pounded Iraqi supply lines and armor concentrations with particular force starting with the second week of Desert Storm.

The Pentagon said Iraqi forces in the Kuwait theater had lost 1,400 of their 4,280 tanks, 800 of their 2,870 armored personnel carriers and 1,200 of their 3,110 artillery pieces.

Although Schwarzkopf declared allied air supremacy as early as Jan. 30, the 24-hour-a-day bombing campaign has not been without cost.

Two A-10 "Warthogs" were shot down on overnight bombing missions against Republican Guard units in northwestern Kuwait, the command announced, and both pilots were listed as missing in action.

An F-16 crashed for unknown reasons during an instrument approach at a U.S. base in Saudi Arabia on return from a mission, killing the pilot, the command said.