A huge bomb explosion followed by a hail of gunfire and grenades killed five U.S. soldiers, the military said Friday. The attack climaxed the deadliest three-month period for the Americans since the war began.

Seven soldiers were wounded in the attack Thursday in the Rasheed district, a mixed Sunni-Shiite area of southern Baghdad where U.S.-led forces recently stepped up pressure on extremists. The commander of U.S. forces in Baghdad suggested the ambush could be part of an escalating backlash by Sunni insurgents.

Those deaths brought to 99 the number of U.S. troops killed this month, according to an Associated Press count. The toll for the past three months – 329 – made it the deadliest quarter for U.S. troops in Iraq since the March 2003 invasion. That surpasses the 316 soldiers killed during November 2004 to January 2005.

Maj. Gen. Joseph F. Fil Jr., who heads U.S. forces in the Iraqi capital, said U.S. casualties had mounted because Sunni extremists are “starting to fight very hard” as U.S. forces press into areas of the capital where militants once had free rein.

“This is a skilled and determined enemy. He’s ruthless. He’s got a thirst for blood like I’ve never seen anywhere in my life,” Fil told reporters. “And he’s determined to do whatever he can.”

During a teleconference with Pentagon reporters, Fil described the Thursday attack as “very violent,” displaying a “level of sophistication that we have not often seen so far in this campaign.”

He said a blast from a “very large” bomb buried deep in the ground triggered the attack, which was followed by volleys of small-arms fire and rocket-propelled grenades. Four soldiers were killed in the attack and a fifth died Thursday night of his wounds, Fil said.

“As far as the assessment, we believe that we are into an area” of south Baghdad “where we’re seeing a very strong al-Qaida cell,” Fil said. “Those areas are now denied to them … They are starting to fight very hard and that’s what we saw yesterday.”

At a separate news conference later, Defense Secretary Robert Gates echoed some of Fil’s remarks about the difficulty of subduing the insurgents who have chosen to fight rather than melt away.

“We’re dealing with … a smart, agile enemy who adjusts his tactics,” Gates said, with Gen. Peter Pace, chairman of the Joint Chiefs of Staff, at his side. He was referring in particular to the insurgents’ ability to kill U.S. troops with enormous homemade bombs often buried in roadways or hidden nearby.

Sunni insurgents have used similar “swarming” tactics for years, mostly in rural areas to the north and west of the capital. Militants have also been burying explosives deep in the ground, making them difficult to detect and triggering them as vehicles pass by.

Such “deep buried bombs” have been especially effective against U.S. vehicles, including Humvees, Bradley fighting vehicles and Strykers, prompting some commanders to shift to foot patrols to avoid losing so many soldiers in a single blast.

U.S. casualties have been rising since President Bush ordered nearly 30,000 more troops to Iraq in a major push to pacify Baghdad and surrounding areas. The goal was to curb the violence so Iraq’s Shiite, Sunni and Kurdish leaders can strike agreements to share power in this fractious country.

But progress toward agreements to share oil wealth, provide a greater political role to the Sunni minority and shore up local governments has been slow because of deep suspicions after four years of bloodshed.

In a hopeful sign, radical Shiite cleric Muqtada al-Sadr called off a July 5 march to a bombed shrine in Samarra north of Baghdad after appeals from the government, which feared Sunni extremists would attack marchers along the way.

Elsewhere Friday, a suicide bomber attacked an Iraqi army post 20 miles north of the capital, killing six soldiers and wounding five others, police said. Two civilians were also killed in a barrage of gunfire.