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

The pipeline blast near Baiji, about 125 miles north of Baghdad, was the latest in a series of attacks by insurgents against infrastructure, possibly to shake public confidence as an interim government prepares to take power June 30.

Meanwhile, gunfire rang out at night in the Shiite holy city of Najaf for the first time since an agreement last week to end weeks of bloody fighting between U.S. soldiers and militiamen loyal to radical cleric Moqtada Sadr. Residents said gunmen attacked a police station near the city's 1920 Revolution Square, and it appeared U.S. 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. First Lt. Amer Jassim speculated that the attackers were firing at Americans but missed.

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