Fourteen Marines and a civilian interpreter were killed in Iraq (search) Wednesday after the amphibious assault vehicle in which they were riding was hit by an improvised explosive, the U.S. military said.

The early morning attack occurred during combat operations near the Iraq-Syria border, raising the number of troops killed this week to 21, the majority of whom were from the same Ohio-based unit that lost six members Monday. One Marine also was wounded in Wednesday's attack, which took place just outside Haditha (search), 140 miles northwest of Baghdad.

"It's very tragic," said Brig. Gen. Carter Ham, speaking at an afternoon Pentagon press briefing.

Details behind the incident were still unclear.

Ham also addressed rumors of a Marine being kidnapped by an Iraqi terrorist group, Ansar al-Sunna, which says a picture on its Web site shows the Marine.

Ham said he has no information to support the claim. "We simply don't know. That is a question certainly that the commanders are asking and are investigating, but the fact of the matter today is while there is a fair amount of speculation as to what could have happened, we just don't know what happened."

The Marines said they have seen the Web video and that sniper rifles terrorists claim to have taken from the six when they were ambushed, as well as a dogtag on display, appear to be U.S.-issued and authentic, but the video's authenticity could not be confirmed.

The Marines said yesterday that the body of one of the Marines killed Monday was found later a few kilometers from the others and that the circumstances were under investigation.

Names of the deceased were being withheld pending notification of next of kin and release by the Department of Defense (search), the military said.

The bombing comes just two days after seven Marines died in the same area during combat with militants. At least 1,820 members of the U.S. military have died since the Iraq war began in March 2003, according to an Associated Press count.

Ham said American forces were mounting simultaneous assaults on a string of towns along the Euphrates River to root out insurgents and cut off their freedom of movement. As insurgents resist, the area has become increasingly dangerous.

An American freelance journalist also was found dead in the southern Iraqi city of Basra, according to the U.S. Embassy in Baghdad. Steven Vincent had been shot multiple times after he and his Iraqi translator were abducted at gunpoint hours earlier, police said.

"U.S. officials will continue to work very closely with British military and local Iraqi forces to find those responsible for this heinous act," the Embassy said in a statement.

The Marines killed Wednesday were assigned to Regimental Combat Team 2, 2nd Marine Division, II Marine Expeditionary Force (Forward), the military said.

"It's the kind of thing that shows the desperation of the adversary," FOX News' Oliver North said in reference to the insurgents' using a roadside bomb instead of engaging troops in a gun battle.

"The further west you go, the wilder the countryside is," he said.

The troops were riding in an amphibious assault vehicle, which is designed to carry troops from ship to shore and on land. It has a road speed of about 45 mph and can carry up to 25. It is not as heavily armored as the Bradley fighting vehicles the Army uses.

The latest losses come two days after seven U.S. Marines were killed in combat in the volatile Euphrates Valley of western Iraq, where American forces are trying to seal a major border infiltration route for foreign fighters.

One of the Marines died in a car bombing in Hit, 85 miles northwest of Baghdad. The other six were killed Monday in Haditha, 50 miles from Hit, while battling insurgents.

The American deaths come as the Bush administration is considering handing more security responsibility to the Iraqis and drawing down forces next year.

At least 39 American service members have been killed in Iraq since July 24 — all but two in combat. In addition, the Iraqi Defense Ministry said that since the beginning of April, more than 2,700 Iraqis — about half of them civilians — had been killed in insurgency-related incidents.

The extremist Ansar al-Sunnah Army claimed responsibility for killing six of the Marines on Monday.

Masked gunmen showed up in the Haditha public market Monday afternoon displaying helmets, flak jackets and other equipment they said were taken from the bodies of the dead Marines.

Fighting has intensified in recent weeks in Haditha, Hit and other dusty towns along the Euphrates River as American forces step up efforts to seal off the approaches to the Syrian border.

The Marines launched a series of operations in the region in May and June in hopes of pacifying the area so Iraqi military and civilian forces could assume effective control. But the insurgents have proven resilient.

“We have seen over the past few months a general decline in the number of improvised explosives attacks ... [but] we are seeing larger amounts of explosives," Ham said. "The changing of techniques over time is a challenge for us."

The U.S. and coalition forces near the Syrian border are aiming to “deny that territory to the insurgents,” Ham said. “It’s an integral component of a larger campaign to deny freedom of movement.”

The area is known as a gateway for insurgents to transfer weapons and money, Ham said.

Iraqi police said Vincent, a writer who had been living in New York, had been staying in Basra for several months working on a book about the city's history.

Vincent and was abducted along with his female translator at gunpoint Tuesday evening, police said. His translator, Nour Weidi, was seriously wounded.

They were seized by five gunmen in a police car as they left a currency exchange shop, police Lt. Col. Karim al-Zaidi said.

As he mulled the relationship among democracy, the Middle East and the war in Iraq, Vincent was at times cynical but more often cautiously optimistic and determined to find signs of hope and progress among the Iraqis he interviewed and befriended.

In an entry from June 12, 2005, he wrote: "The people here desperately need — and deserve — law and order, a sense that justice can prevail against malevolent powers stalking their nation."

In an opinion column published July 31 in The New York Times, Vincent wrote that Basra's police force had been heavily infiltrated by members of Shiite political groups, including those loyal to radical cleric Muqtada al-Sadr.

Vincent quoted an unidentified Iraqi police lieutenant as saying that some police were behind many of the assassinations of former Baath Party members that have taken place in Basra.

Vincent's body was discovered on the side of the highway south of Basra later. He had been shot in the head and multiple times in the body, al-Zaidi said.

According to the New York-based Committee to Protect Journalists, at least 46 journalists and 20 media support workers have been killed while covering the war in Iraq since March 2003. Insurgent actions are responsible for the bulk of deaths.

The Vienna, Austria-based media watchdog International Press Institute condemned Vincent's killing and urged Iraqi authorities to conduct a speedy and thorough investigation.

The death underscored how "Iraq continues to be the most dangerous country in the world in which to work as a journalist," the group said.

The Associated Press contributed to this report.