An explosion damaged part of the main pipeline running from Iraq's northern oil fields on Thursday, forcing a reduction in the amount of oil available for export.

It was unclear whether the explosion along a section of pipeline near the city of Hadeetha (search), 125 miles northwest of Baghdad, was caused by saboteurs, a senior Oil Ministry official said on condition of anonymity.

He said the explosion ripped open part of the main pipeline linking the northern oil fields to the al-Doura oil refinery and the Mussayab power plant (search). The oil in the pipeline was earmarked for domestic use.

To maintain domestic supplies, the official said exports from the southern oil fields will be reduced by 80,000 barrels a day to make up for the shortage from the northern oil fields.

There have been many attacks on pipelines in the region, complicating the American rebuilding effort in Iraq, which depends on oil revenue.

L. Paul Bremer, the U.S. civilian administrator for Iraq, has said the country is losing $7 million daily because of the closure of the export pipeline to Turkey. In September, the line reopened for three days for the first time after the war. Three bomb blasts along the line forced its closure.

Iraq is exporting an average of 1 million barrels of oil a day, all of it coming from the southern oil fields.

In Tikrit, meanwhile, a 4-year-old Iraqi girl was killed Thursday when a bomb exploded just outside the main U.S. Army base. Her 12-year-old sister was critically wounded, U.S. officials said.

U.S. officials said they believed the bomb was intended for two U.S. Bradley armored vehicles that had passed down the same road minutes before the blast.

In the southern city of Basra, an Iraqi doctor, Haidar al-Baaj, was shot in the back of the head and killed as he was entering his clinic, hospital officials said Thursday.

Al-Baaj, 48, was recently promoted to the post of director of the Educational Hospital in Basra (search), the officials said. The officials and members of al-Baaj's family said he had been threatened over the past two months for cooperating with authorities of the U.S.-run coalition.

British military spokesman Capt. Hisham Halawi confirmed a doctor was killed on Wednesday but did not provide further details.

A total of 194 U.S. soldiers have been killed by bombings, ambushes and other hostile incidents since President Bush declared an end to major combat operations May 1.

Many have been killed by roadside bombs, which insurgents build from explosives believed to come from Iraqi army depots looted after the collapse of Saddam Hussein's regime in April.

Brig. Gen. Robert L. Davis told reporters between 600,000 and 1 million tons of Iraqi munitions remain unaccounted for, though he could not say how much of that had been used in attacks against U.S troops.

Meanwhile, Iraqi security officials said a woman with explosives tied to her belt was arrested Tuesday as she tried to enter a Finance Ministry building, apparently to carry out a suicide attack.

The woman, who appeared to be in her late 40s, was stopped at the checkpoint at the entrance to the building, where guards found the explosives, the officials said Thursday, speaking on condition of anonymity.

The incident occurred in a building next to the Iraqi Central Bank on the same day as a suicide bomber tried to blow up the Turkish Embassy, killing himself and wounding more than a dozen others.

On Thursday, troops of the 1st Battalion, 22nd Infantry Regiment held a memorial service for two comrades killed in attacks this week.

Spc. James Powell, 26, of Ohio, died Sunday when his Bradley armored vehicle struck a land mine near Beiji, 30 miles north of Tikrit. Spc. Donald L. Wheeler of Concord, Mich., was killed in an attack Monday in Tikrit, when a rocket-propelled grenade hit his vehicle.

The soldiers' helmets and name tags were placed over their rifle butts, next to their boots on a small podium adorned with the U.S. flag and the regimental banner.