Fallujah, Iraq ? U.S. warplanes strafed gunmen in Fallujah on Wednesday, and more than 100 guerrillas with rocket-propelled grenades pounded a lone Marine armored vehicle lost in the streets — a sign of heavy battles ahead if Marines resume a full assault on this besieged city.

Both Marines and insurgents were fortifying their positions in preparation for more fighting.

In abandoned homes a few blocks into the city, Marines punched bricks out of walls to make holes through which to fire, and knocked down walls between rooftop terraces to allow movement from house to house without descending to the street. They spread shards of glass across doorsteps to hear the boot of an approaching insurgent.

Insurgents were also organizing. Gunmen were believed to be digging tunnels under the houses they hold to allow them to move without being targeted by Marine snipers, Marines said.

A 4-day-old truce was crumbling amid nightly battles in which gunmen in larger groups have been attacking U.S. troops with increasing sophistication. Wednesday night the fighting began again, with AC-130 gunships over the city battering targets below.

The top Marine commander in the Fallujah area suggested time for negotiations was running out before U.S. forces call off their halt in offensive operations.

“I don’t forecast that this stalemate will go on for long,” said Maj. Gen. James N. Mattis, commander of the 1st Marine Division. “It’s hard to have a cease-fire when they maneuver against us, they fire at us.”

Tuesday night, insurgents launched near simultaneous attacks on several positions of a company of Marines controlling a few blocks in the city’s northeast. In one attack, the gunmen sent up flares to light up the American position, then unleashed heavy, continuous gunfire, Marines said.

U.S. Marines from the 1st Battalion 5th Marine Regiment unload a large cache of weapons, including homemade multiple rocket launchers, at right, in Fallujah, Iraq. The Marines on Wednesday found two separate caches of weapons during searches of buildings in Fallujah.

In a five-hour battle the same night, one of two armored vehicles sent to resupply a front-line Marine position got lost during an ambush and ended up nearly half a mile inside the southern part of city.

The vehicle, with 20 Marines inside, came under an even larger ambush. At least 100 gunmen opened fire with rocket-propelled grenades, hitting it at least 10 times, knocking out its communications and its engine and paralyzing it.

“They’ve been preparing for this the whole time. … We definitely stumbled into the wasp nest,” said Captain Jason Smith, who was at the position meant to be resupplied.

The Marines in the armored vehicle fled into a nearby building, where they waited to be rescued. They threw back grenades that insurgents tossed over the wall and listened to gunmen whisper outside.

A rescue force, backed by four tanks, wandered the streets in search of the beleaguered vehicle, finding it by following black smoke. “We were firing in a 360-degree radius,” said Lt. Joshua Glover, part of the team that reached the vehicle. While F-15 warplanes strafed the area for cover, the stricken armored vehicle was hooked to a tank and dragged away.