BAGHDAD, Iraq - A mortar attack killed four employees of a British security firm and wounded 15 others in the Baghdad's Green Zone, a fortified area that houses the U.S. and Iraqi leadership, the company and British officials said today.

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 O'Brien, spokesman for the London-based Global Risk Strategies, 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.

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

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

Meanwhile, U.S. forces found 13 more bodies in and around the northern city of Mosul, the military said today, bringing to 35 the number of corpses discovered in the past week in the area shaken by an insurgent uprising.

Eleven of the 35 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.

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 Thursday 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.

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 today.

Lt. Gen. John . Sattler, commanding general of the 1st Marine Expeditionary Force, 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 weeklong U.S.-led assault that began Nov. 8. Sattler said about 50 percent of the houses in the city have already been cleared.

"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," he said.

In the southern city of Basra, Iraqi forces arrested four insurgents who said they were planning attacks against coalition bases and police stations there, officials said today, a day after a joint British-Iraqi operation netted three dozen men in the area.

Iraqi National Guardsmen arrested the four after a brief gunfight at the Al-Yarmouk Hotel. Three of the men came from Fallujah and the fourth from Samarra, according to an Iraqi National Guard official who spoke on condition of anonymity.

The four men told Iraqi officials they were planning a series of attacks in southern Basra, which is the headquarters for some 8,500 British troops, in an attempt to relieve the U.S. military pressure on Fallujah.

One of the insurgents was injured during the gunfight and weapons were found in their hotel room, the Iraqi official said.

The arrests followed a larger operation Thursday where Iraqi National Guards and police commandos, backed by coalition forces, raided several locations in the town of Zubayr, just west of Basra, said British spokesman Maj. Charlie Mayo.

A total of 36 people were detained, all of them from outside the Basra area, along with quantities of weapons, ammunition and drugs, he said.

Iraqi officials said the group included Iraqis, Afghans, and other Arab foreign fighters, though there was no breakdown.