Insurgents killed 14 U.S. Marines and their civilian interpreter in a roadside bomb attack in northwestern Iraq Wednesday, just one day after the military announced that seven U.S. Marines were also killed in Iraq's Anbar province, the military said.

The military said the Marines were killed Wednesday morning when their amphibious assault vehicle was attacked just south of the northwestern city of Haditha, which is 125 miles northwest of Baghdad. One Marine was also wounded in the attack, the military said.

Six of the Marines killed Monday were slain in the same area, the military said, which is a known hiding place for insurgent fighters. The seventh Marine was killed by a suicide bomber about 45 miles away.

At a Pentagon briefing in Washington, Brig. Gen. Carter Ham, deputy director of operations at the Joint Chiefs of Staff, described the insurgents as a "very brutal, lethal and adaptive enemy" operating in towns along the Euphrates River to the Syrian border.

"They are dangerous and they certainly have a capability," Ham said. He said U.S. Marines had been working simultaneous missions in a number of towns along the river in an effort to limit the insurgents' ability to move freely and establish safe havens. Asked whether the recent attacks meant the insurgents now had the ability to operate freely in the area, Ham replied, "I think not."

"What we're seeing here is a concerted effort to assert control, ultimately Iraqi control, in those towns and there's resistance that is coming from the insurgents in those towns," Ham said.

The killings brought the number of U.S. military personnel killed in Iraq since the March 2003 start of the war to 1,816, according to figures released by the Pentagon.

The back-to-back attacks marked one of the bloodiest two-day periods for American forces in months and came after weeks in which insurgent attacks have mostly targeted Iraqi civilians, officials and security forces.

All 21 Marines were killed in Anbar province, which U.S. commanders call a crossroads and hiding place for insurgent fighters, arms and money entering Iraq from neighboring Syria.

"When is this going to end?" demanded a shopkeeper in a Baghdad neighborhood where a suicide bomber attacked a U.S. military convoy Tuesday, wounding 22 Iraqi civilians and an unspecified number of American troops.

As a bystander showed off a cardboard box containing the bomber's body parts, the shopkeeper asked: "When are we going to have quiet again and live like normal people?"

Since spring, the Marines have mounted at least a half-dozen major campaigns against the insurgents in the west. A U.S. military statement said six of the Marines who died Monday also died outside Haditha, a city believed to be a stronghold for foreign insurgents.

The six were conducting "dismounted" operations, meaning they were out of their armored vehicles, when insurgents fired on the men, killing at least five, the statement said.

The body of the sixth Marine was recovered one or two miles away, the military said, without saying how he died. At the briefing in Washington, Pentagon officials said they had no information on why the Marine was found in a separate location.

The military released the names of the six slain Marines on Wednesday. They were: Cpl. Jeffrey A. Boskovitch, 25, of Seven Hills, Ohio; Lance Cpl. Roger D. Castleberry Jr., 26, of Austin, Texas; Sgt. David J. Coullard, 32, of Glastonbury, Conn.; Lance Cpl. Daniel N. Deyarmin Jr., 22, of Tallmadge, Ohio; Lance Cpl. Brian P. Montgomery, 26, of Willoughby, Ohio, and Sgt. Nathaniel S. Rock, 26, of Toronto, Ohio.

The seventh Marine killed Monday by a suicide bomber was Sgt. James R. Graham III, 25, of Coweta, Okla., the military said.

Although bombs in Iraq have occasionally killed a handful of Marines in a single attack, it is unusual for five to die from small-arms fire.

Marines outside their armored vehicles generally wear body armor and carry weapons at all times, except when sleeping. U.S. military statements gave no other details, saying the attack was under investigation.

News agencies said one of Iraq's most violent Islamic guerrilla groups, the Ansar al-Sunna Army, asserted responsibility for the killings.

In another incident, an American journalist was shot and killed in the southern city of Basra, the Reuters news agency reported.

Steven Vincent, author of "In the Red Zone: A Journey Into the Soul of Iraq," was working on another book on Iraq when he was killed by unknown assailants.

Separately, Iraq, claimant to the world's second-largest oil reserves, announced fuel rationing during a summer of miles-long gas lines. The reasons for the rationing cited by government officials and analysts matched a litany of Iraq's problems: insurgent attacks, corruption, decayed infrastructure and mismanagement.

In Baghdad, Iraqis reacted with frustration to the announcement of the impending fuel rationing.

While second only to Saudi Arabia in crude reserves, Iraq imports hundreds of millions of dollars of refined petroleum products. The country's own refineries are dilapidated and inadequate.

Frequent insurgent attacks disrupt supply, and smugglers divert tons of gasoline to neighboring countries. With electrical outages almost round-the-clock this summer in Baghdad, generators also eat up supplies of diesel fuel and gas.

Rationing was one of a list of measures that Oil Minister Ibrahim Bahr Uloom announced to try to improve supply. Others included an anti-corruption campaign and a promise to keep some fuel stations open 24 hours a day to deal with the miles-long lines.

In a two-mile-long line on Tuesday, a university student, Saman Mohammed, said the measures would only make Iraq's more readily available black-market gas more expensive.

He wondered why Iraqis saw so little of their country's fuel riches. "Our country is collapsed and became like fallen prey, and the beasts are eating it," Mohammed said. "But eventually, it is our fault. Everyone in this country should cooperate in order to build this country."

Special correspondents Khalid Saffar and Bassam Sebti contributed to this report. Deane reported from Washington.