WASHINGTON — More American troops were killed in combat in Iraq over the past four months — at least 334 through Jan. 31 — than in any comparable stretch since the war began, according to an Associated Press analysis of casualty records.

Not since the bloody battle for Fallujah in 2004 has the death toll spiked so high.

The reason is that U.S. soldiers and Marines are fighting more battles in the streets of the Iraqi capital, Baghdad, and other cities. And while hostile forces are using a variety of weaponry, the top killer is the roadside bomb.

In some respects it is the urban warfare that U.S. commanders thought they had managed to largely avoid after U.S. troops entered Baghdad in early April 2003 and quickly toppled the Saddam Hussein regime.

And with President Bush now sending thousands more U.S. troops to Baghdad and western Anbar province, despite opposition in Congress and the American public’s increasing war weariness, the prospect looms of even higher casualties.

The shadowy insurgency has managed to counter or compensate for every new U.S. military technique for defeating roadside bombs, which over time have proliferated and grown increasingly powerful. The U.S. has spent billions trying to counter that threat, and the Bush administration in its budget 2008 request to Congress this week asked for another $6.4 billion to find more effective defenses against it.

The Pentagon’s terse death announcements only begin to tell the story:

• Sgt. Corey J. Aultz, 31, of Port Orchard, Wash., and Sgt. Milton A. Gist, 27, of St. Louis, died Jan. 30 in Ramadi, the capital of Anbar province, of wounds from an improvised bomb that detonated near their vehicle.

• Three days earlier, three soldiers — one just 19 years old — were killed by a roadside bomb in Taji, just north of Baghdad. And a week before that, four soldiers, from towns in the four corners of this country — Florida, New Hampshire, Oregon and California — were killed by a roadside bomb not far from Fallujah.

The increasingly urban nature of the war is reflected in the fact that a higher percentage of U.S. deaths have been in Baghdad lately. Over the course of the war, at least 1,142 U.S. troops have died in Anbar province, the heart of the Sunni Arab insurgency, through Feb. 6, according to an AP count. That compares with 713 in Baghdad. But since Dec. 28, 2006, there were more in Baghdad than in Anbar — 33 to 31.

The surge in combat deaths comes as the Pentagon begins adding 21,500 troops in Iraq as part of Bush’s new strategy for stabilizing the country. Most are going to Baghdad, but some are being sent Anbar.

With the buildup, U.S. forces will be operating more aggressively in Baghdad as they try to tamp down sectarian bloodshed, a tactical shift that senior military officials say raises the prospect of even higher U.S. casualties.

“There’s clearly going to be an increased risk in this area,” Adm. William Fallon, Bush’s choice to be the next commander of U.S. forces in the Middle East, told his Senate confirmation hearing last week.

Risk is already extraordinarily high from known threats, including roadside bombs.

The frustrating fact about the hunt for a solution to the roadside bomb is that the Americans have improved their ability to find and disarm them before they detonate, and they have outfitted troops in better body armor. But the insurgents still manage to adjust: new tactics in planting the bombs, new, more powerful explosives, different means of detonating them and, amazingly, a seemingly endless supply of materials.

Defense Secretary Robert Gates said Friday that 70 percent of U.S. casualties are caused by such bombs. He said that lately Iran, allegedly in league with renegade Shiite groups in southern Iraq, has had a hand in supplying a more lethal version so powerful it can destroy a U.S. Abrams battle tank, which is shielded with heavy armor.

On Jan. 22, Army National Guard Spc. Brandon L. Stout, 23, of Grand Rapids, Mich., was killed by one of those more powerful bombs, known as an explosively formed projectile, that went off near his vehicle in Baghdad. A week earlier, four soldiers were killed by a roadside bomb in the northern city of Mosul.

It is not possible to fully track the trend in bomb-caused deaths by month. The U.S. military considers such information secret because it is considered potentially useful to the insurgents and their backers. Also, the Marines do not announce the specific cause of any of their combat deaths, whereas the Army does.

Hostile forces also have had more success lately shooting down U.S. helicopters, Gen. Peter Pace, chairman of the Joint Chiefs of Staff, acknowledged Tuesday. He said four U.S. helicopters in recent weeks have been shot down by small arms fire, including a Black Hawk in which all 12 National Guard soldiers aboard were killed.

What’s more, there have been troubling new twists to some other attacks, including the sneak attack in Karbala that killed five U.S. soldiers; four of them were abducted and executed by unknown gunmen. U.S. officials say they are studying the possibility that Iranian agents either planned or executed that Jan. 20 attack.