Baghdad, Iraq ? Saboteurs blew up a key northern oil pipeline Wednesday, forcing a 10 percent cut on the national power grid as demand for electricity rises with the advent of Iraq’s broiling summer heat.

Meanwhile, gunfire rang out Wednesday night in the Shiite holy city of Najaf for the first time since an agreement last week to end weeks of bloody fighting between American soldiers and militiamen loyal to radical cleric Muqtada al-Sadr.

Residents said gunmen attacked a police station near the city’s Revolution of 1920 Square, and it appeared American troops were not involved.

Clashes persisted Wednesday around Fallujah, a rebellious Sunni Muslim city west of Baghdad. Four members of an Iraqi force in charge of the city since April were wounded when a mortar round exploded. 1st Lt. Amer Jassim speculated the attackers were firing at Americans but missed.

The pipeline blast near Beiji, 155 miles north of Baghdad, was the latest in a series of attacks by insurgents against infrastructure targets, possibly to shake public confidence as a new Iraqi government prepares to take power June 30.

The attack on the pipeline — which carries fuel to the Beiji power station, one of Iraq’s largest — forced a 10 percent cutback in the country’s 4,000-megawatt production, Assem Jihad, an Oil Ministry spokesman, told Dow Jones Newswires.

The U.S.-run coalition had made its ability to guarantee adequate electricity supplies a benchmark of success in restoring normalcy to Iraq. However, sabotage and frayed infrastructure have impeded efforts to eliminate power outages, especially in the capital.

More than a year after the occupation began, power cuts are common nationwide, in some places topping 16 hours a day. Demand is rising with the advent of summer, with temperatures already topping 100 degrees.

Elsewhere, Polish authorities said an explosion that killed six European soldiers — two Poles, three Slovaks and one Latvian — south of Baghdad on Tuesday was caused by a mortar attack rather than an accident as first reported.

Gen. Piotr Czerwinski, the head of a special investigating commission, said he suspected that Saddam Hussein loyalists were responsible for the deaths — the first in Iraq for the small Slovak and Latvian contingents.

In other developments:

l In Rome, three Italians returned home Wednesday, a day after they and a Polish hostage were freed by coalition forces. Kidnappers had held the Italians for two months.

l A group holding two hostages — a Turk and an Egyptian — threatened to kill the captives after Friday prayers unless their home governments condemn U.S. actions in Iraq. The threat was made in a statement distributed in Fallujah.

An oil pipeline burns in the desert near Beiji, Iraq, about 155 miles north of Baghdad. Saboteurs blew up the pipeline Wednesday, forcing a 10 percent cut in output for the national electricity grid, Iraqi officials said.