About 1,100 U.S. soldiers and Marines were wounded in Iraq during August, by far the highest combat injury toll for any month since the war began and an indication of the intensity of battles flaring in urban areas.

U.S. medical commanders say the sharp rise in battlefield injuries reflects more than three weeks of fighting by two Army and one Marine battalion in the southern city of Najaf. At the same time, U.S. units frequently faced combat in a sprawling Shiite Muslim slum in Baghdad and in the Sunni cities of Fallujah, Ramadi and Samarra, all of which remain under the control of insurgents two months after the transfer of political authority.

"They were doing battlefield urban operations in four places at one time," said Lt. Col. Albert Maas, operations officer for the 2nd Medical Brigade, which oversees U.S. combat hospitals in Iraq. "It's like working in downtown Detroit. You're going literally building to building."

The sharp rise in wounded was, for the first time, accompanied by a far less steep climb in battlefield fatalities. Since the start of the war in March 2003, 979 U.S. troops have died in Iraq and almost 7,000 have been wounded. Until last month, however, the monthly tallies of fatalities and wounded rose and fell roughly in proportion.

In August, 66 U.S. service personnel were killed in Iraq, according to the Defense Department. The toll was the highest since May, when 80 fatalities were recorded. But it was well below the 135 U.S. combat deaths in April, when a sporadic guerrilla war that had largely been confined to the so-called Sunni Triangle north and west of Baghdad spread to cities across the previously quiescent Shiite Muslim belt in southern Iraq. The U.S. military does not routinely release the reported number of Iraqi casualties and wounded.

Commanders said they had no immediate concrete explanation for why the number of wounded increased so sharply without a comparable rise in combat deaths.

"All I know is I've got more patients here," said Col. Ryck Beitz, commander of the 31st Combat Support Hospital in Baghdad, which admitted 425 patients last month, a new high.

One possible explanation lay in the brawn some units brought to the fight in crowded city centers. In Najaf, for example, two of the three U.S. battalions squaring off in close quarters against a Shiite militia were categorized as "heavy armored." Army officers said their Abrams tanks and Bradley Fighting Vehicles not only offer substantial protection from incoming fire, but also answered with immediate and overwhelming large-caliber salvos.

"We've been given the best tools in the world for waging war," said Maj. Tim Karcher of the 2nd battalion of the 1st Cavalry Division's 7th Regiment. In two weeks of almost constant combat, the heavily armored battalion sustained several injuries, but not a single fatality, as it fought its way through Najaf's crowded old city.

Lives surely were also saved by the proximity of the fighting to a nearby combat hospital; the forward surgical station at Babylon is a short Black Hawk helicopter hop from Najaf.

Maj. Dellone Pascascio, who compiles tallies of U.S. wounded across Iraq, said injuries sustained in conventional fighting may tend to be slightly less severe than those inflicted by the improvised explosive devices planted along roadsides that continue to kill and maim U.S. forces by spraying shrapnel upward.

There were also indications that troops might have suffered more severe wounds in August than in previous months.

At the Baghdad hospital, staff members are accustomed to seeing the most severely injured soldiers and Marines. The hospital, the only one in Iraq where the military's brain and eye surgeons work, handles the worst head wounds. Normally, perhaps half the patients who come to the emergency room qualify as "acute" cases, a term that indicates severity and urgency.

"A soldier who comes in and is almost bleeding to death will require more care than someone who is just shot with a bullet," Beitz explained.

In August, however, the rate of acute cases jumped to three of four ER patients.

"It was intense," said Lt. Col. Greg Kidwell, who oversees the emergency room at the hospital.

Capt. Neil Taufen, an emergency room physician, said the pace was all the more striking because it came after a quiet stretch.

"July was just dull, and it was like: Everything's going to be all right. And then Najaf fired up, and it was just like nothing had ever changed," said Taufen, of Fort Sill, Okla.

Najaf and the neighboring town of Kufa, about 90 miles south of Baghdad, have been quiet since a peace deal was brokered in late August by Grand Ayatollah Ali Sistani. Last week, an informal cease-fire also took hold in Sadr City, the Shiite slum that is the main stronghold of junior cleric Moqtada Sadr and his Mahdi Army militia, which fought in Najaf.

But U.S. forces continued to clash with Sunni Muslim insurgents and foreign-born fighters west and north of Baghdad. Twenty-six Marines were killed during August in Anbar province, which takes in Fallujah and Ramadi and extends across the western desert to the Syrian border. Insurgents hold sway in both cities and routinely attack U.S. patrols.

"It's always kind of a smoldering fight out there," Kidwell said.

Parts of Baghdad also remain combat zones.

Propped on pillows in a ward of the Baghdad combat hospital Saturday afternoon, Spec. Christopher Riang, 19, wore a zipper of surgical staples up his abdomen after a nasty surprise the night before off the capital's hostile Haifa Street.

"I yelled 'grenade!' and made sure the Iraqi translator took off," he said, describing the overnight ambush that left him with a belly full of steel shards. "Then I took off. I guess I couldn't outrun the grenade."

The interpreter was also injured, as were four other 1st Cavalry soldiers caught in the alley when grenades began raining down.

"Almost everybody took shrapnel," said Capt. Chris Ford, the company commander. Three soldiers were injured lightly enough to return to duty after treatment, as are about 45 percent of U.S. forces wounded in Iraq. Two others needed medical evacuation. The interpreter went home.

"Basically, we had to fight our way out of that alley," Ford said. Bradleys came to help, he said, but most of the patrolling in the largely hostile neighborhood is conducted on foot.

"It's a labyrinth," Ford said. "And it's conducive to their kind of fighting."

More and more often, children are lobbing the grenades, Ford said. Insurgents offer boys of 10 or 12 years old $150 to toss a grenade at a U.S. patrol, the captain said.

"For the longest time, we've had a good relationship with the children," Ford said. "Now this. Who enjoys putting a bead on a kid?

"Nobody. That's why they paid them."