BAGHDAD, Iraq – Fourteen American Marines were killed yesterday when a huge bomb destroyed their lightly armored vehicle, hurling it into the air in a giant fireball in the deadliest roadside bombing suffered by American forces in the Iraq war.

A civilian translator also was killed and one Marine was wounded. The victims were from the same Ohio-based Reserve unit as six members of a Marine sniper team killed on Monday in an ambush claimed by the Islamic extremist Ansar al-Sunnah Army.

The deaths brought to 23 the number of Marines killed in the past week in fighting along the volatile Euphrates Valley of western Iraq and marked one of the bloodiest periods for American forces in months. In all, 44 American service members have died in Iraq since July 24 – all but two in combat.

A Marine officer, speaking on condition of anonymity for security reasons, said the attack occurred as troops were traveling in an armored amphibious vehicle to assault insurgent positions around a village near the Haditha dam, a longtime way station for foreign fighters infiltrating Iraq from Syria.

Suddenly, a thunderous explosion rang out and the vehicle flipped over in a fireball, he said. The surviving Marine scrambled from beneath the overturned vehicle, the officer said.

The Marines killed yesterday were assigned to the 3rd Battalion, 25th Marines based in Brook Park, a Cleveland suburb, and attached to the Regimental Combat Team-2. Nine of them were from a single smaller unit in Columbus.

President Bush lamented the deaths of the 14 Marines, calling the attack a “grim reminder” America is still at war.

“These terrorists and insurgents will use brutal tactics because they’re trying to shake the will of the United States of America. They want us to retreat,” Mr. Bush told some 2,000 lawmakers, business leaders, and public policy experts in Grapevine, Texas.

The heavy loss of life cast new attention on a longtime Marine complaint – the lack of protection provided by their armored amphibious vehicles, or AAVs. Although fast and maneuverable, the vehicles have armor plating that is lighter than those used by the Army.

Moreover, American commanders have warned that while insurgent bombings have been declining in number, they have been increasing in power and sophistication.

“This is a very lethal and unfortunately very adaptable enemy we are faced with,” said U.S. Army Brigadier General Carter Ham, a Pentagon staff officer and former commander of American forces in Mosul.

Marines have been fighting for months in a string of towns along the Euphrates to try to seal a major infiltration route for foreign fighters slipping into Iraq from Syria. Late yesterday, insurgents fired two mortars at Marine positions near Haditha. Moments later, American warplanes could be heard mounting counterattacks, residents said.

The Marines stepped up operations in May in hopes of pacifying the area so Iraqi military and civilian forces could assume effective control. However, government authority in the heavily Sunni Arab region is tenuous.

American officials have long complained that American forces seize Sunni areas only to have Iraqi authorities lose them again to the insurgents once American troops leave.

Yesterday, the Web site of the Ansar al-Sunnah Army posted photographs from Monday’s attack on the Marine sniper team. One picture shows a bloody, battered body wearing Marine camouflage trousers. Another shows two hooded gunmen standing in front of several rifles, apparently taken from dead Marines.

In a statement accompanying the photos, Ansar al-Sunnah said the insurgents lured the Marines out of their base and ambushed them.

“The intention was to capture them alive, but they opened fire on the mujahedeen,” the statement said. “The heroes slaughtered those who were still alive … except for one, who begged the mujahedeen for his life. They captured him and he is in our hands.”

At the Pentagon, General Ham said no Marines were missing and believed captured.

In the Cleveland suburb where the battalion was based, Brook Park, businesses tied red, white, and blue ribbons on their doors. A bouquet of roses was placed at the gate of the Marine headquarters, an old brick schoolhouse.

Among the six killed Monday was an aspiring police officer who planned to set a wedding date with his girlfriend when he returned home this fall, Corporal Jeffrey Boskovitch, 25, of North Royalton, Ohio.

In Basra, 340 miles southeast of Baghdad, an American freelance writer was found dead late Tuesday – the first American journalist slain in Iraq since the American-led invasion. Steven Vincent of New York was shot multiple times hours after he and his Iraqi translator were abducted at gunpoint, police said.

The translator, Nour Weidi, was seriously wounded. Five gunmen in a police car abducted them as they left a currency exchange shop Tuesday evening, police Lieutenant Colonel Karim al-Zaidi said. Vincent had been in Basra for several months working on a book about the city’s history.