Two U.S. Marines were killed Friday during house searches in Fallujah, the epicenter of insurgent resistant that remains dangerous despite U.S. assertions that the military campaign there was a success.

Besides the American casualties, three enemy fighters were also killed in Fallujah (search) as U.S. forces went house-to-house to clear out remaining gunmen, according to U.S. military officials. Fifty percent of the city is now clear of insurgents, officials said.

"We will continue to clear out houses till every one is secure. We’ve taken more and more of their safehouses. They’re running out of places to hide,” said Lt. Gen. John F. Sattler, commanding general of the 1st Marine Expeditionary Force (search).

Elsewhere in Iraq on Friday:

— In the northern city of Mosul, 21 bodies more were found, bringing the total fatality count for that city to more than 40 this week.

— Several Iraqi political parties signed a petition on Friday calling for the planned Jan. 30 elections to be delayed by up to six months.

— A militant group claimed responsibility on its Web site for a mortar attack that killed four employees of a British security firm Thursday in Baghdad's Green Zone, a fortified area that houses the U.S. and Iraqi leadership.

In Fallujah, insurgents ambushed U.S. troops as they entered a home during house-to-house searches in the former rebel bastion, killing two Marines and wounding three others, the U.S. military said Friday.

Sattler said the Marines responded with gunfire, killing three rebels hiding inside.

U.S. troops have continued clearing operations in Fallujah, which came under a massive week-long U.S.-led assault that began Nov. 8. Sattler said about 50 percent of the houses in the city have already been cleared.

Bodies Discovered in Mosul

Eleven of the bodies found in Mosul — site of an insurgent uprising this week — have been identified as members of the Iraqi security forces, who have been targeted by insurgents. The others have not been identified.

"It's a continued campaign of threats, intimidation and murder by insurgents to spread fear into the public. Their campaign has been directed at what appears to be Iraqi security forces," said Lt. Col. Paul Hastings, a spokesman with Task Force Olympia (search).

U.S. forces patrolling Mosul and nearby Tal Afar on Thursday morning found nine bodies on the western side of Mosul, he said. Two more bodies were found in the city later in the day.

In Tal Afar, one Iraqi National Guard soldier was found dead while a second body discovered in a different location was unidentified.

The military late Thursday had reported the discovery of another two bullet-riddled bodies in western Mosul. U.S. and Iraqi troops were hit by mortars, rocket-propelled grenades and small arms fire as they were retrieving the bodies, according to Lt. Col. Eric Kurilla of the Army's 1st Battalion, 24th Infantry Regiment.

The U.S. military spotted at least one team of insurgents firing at them. One Iraqi National Guardsman suffered minor injuries, according to Kurilla.

Mosul, Iraq's third-largest city, was the site of a mass insurgent uprising in apparent support of Fallujah guerrillas following the U.S.-led assault on that rebel stronghold on Nov. 8. In the wake of the mass attacks, U.S. and Iraqi forces were sent in to retake parts of the city but insurgents have managed to hit back.

Twenty other bodies have been found in Mosul since last Thursday. At least 10 of the bodies — nine of them shot execution-style — belonged to the Iraqi regular army, based at the al-Kisik military base about 31 miles west of Mosul, near Tal Afar. Four of the bodies found were decapitated.

Green Zone Attacks Kills 4

A mortar attack that killed four Nepalese security contractors of a British firm and wounded 15 others in Baghdad's Green Zone, a fortified area that houses the U.S. and Iraqi leadership, was the work of the extremist group Ansar al-Sunnah Army, its Web site claimed Friday.

The same group claimed responsibility for massacring 12 Nepalese construction workers last August.

Britain's Foreign Office said the dead were former Gurkhas, renowned Nepalese soldiers.

"The mortar landed in their camp," said a Foreign Office official on condition of anonymity

Tim J. O'Brien, spokesman for the London-based Global Risk Strategies (search), said the attack took place Thursday, when multiple explosions were heard and black smoke was seen rising from the fortified zone on the western bank of the Tigris River.

"There was an incident yesterday. We lost four people and had 12 to 15 who were injured," he said. "We can't confirm what this incident actually was until we go through internal investigations."

O'Brien declined to provide the identities of the victims, but said that none of those killed were American.

Global Risk Strategies is a London-based firm that provides security in countries including Iraq and Afghanistan.

Question of Elections

Despite the recent rash of violence, Sattler vowed the country will be safe in time for next January’s nationwide elections. U.S. Navy Secretary, Gordon England, who’s in Iraq now, said he believes they can have a successful election here in January.

"We had a similar situation in Afghanistan. Prior to elections there was a lot of insurgency in Afghanistan but after elections the country's made great. Great progress and I believe the same thing will happen here in Iraq," he said.

Other disputes are shrouding the upcoming election as well. Slated to take place on Jan. 30, Sunni politicians are asking officials to postpone the vote in an effort to persuade Sunni clerics from boycotting the event, for which voter registration is already taking place in various spots around the country.

On Friday, the Patriotic Union of Kurdistan (PUK) and Kurdistan Democratic Party (KDP), the main Kurdish parties in northern Iraq, signed the petition calling for an election delay. Also signing was a group led by former presidential candidate Adnan Pachachi.

The government has already ruled out any delay.

FOX News' David Piper and The Associated Press contributed to this report.