Insurgents assassinated a senior Iraqi security official in the rich northern oil fields Wednesday and blew up another key pipeline in the south, closing the spigot of Iraq's vital petroleum exports through the Persian Gulf in attacks that could cost the country tens of millions of dollars a day in oil revenue.

The violence was the latest in a campaign of car bombings, roadside explosions and other assaults that have generated a climate of fear among many Iraqis instead of the national pride U.S. officials were hoping for as the handover of limited authority to an interim Iraqi government approaches on June 30. In addition to targeting U.S. soldiers and foreigners working here, insurgents have increasingly directed their fire at middle-ranking Iraqi officials in the country's U.S.-sponsored provisional administration, from police officers to professors and bureaucrats.

A roadside bomb blew up Wednesday near Ramadi, 60 miles west of Baghdad, killing one Iraqi policeman and wounding five Iraqi civilians, the U.S. military said. Marines took into custody three Iraqi civilians who tried to flee the site and six U.S.-trained Iraqi Civil Defense Corps policemen suspected of participating in the bombing, a military spokeswoman said.

Three U.S. soldiers were also killed Wednesday and at least 20 people were wounded when a rocket crashed into a U.S. base near Balad, 50 miles north of Baghdad, the U.S. military reported. A statement from the U.S. command said aircraft and other forces were called in but failed to find those who fired the rocket.

An attack Tuesday, this one by gunmen who fired from an overpass on a convoy of vehicles driving near Baghdad International Airport, killed three Iraqi security workers contracted by the U.S.-run Coalition Provisional Authority, the U.S. command reported Wednesday.

Ghazi Talabani, who was chief of security for Northern Oil Co., was gunned down as he drove to work in Kirkuk, a city at the center of Iraq's northern oil fields, police told reporters. Talabani was related to Jalal Talabani, leader of the Patriotic Union of Kurdistan, one of two main parties that, with their militias, have run Iraq's Kurdish area as a semi-independent zone under U.S. protection for more than a decade.

Talabani was the third senior official assassinated in the last week in Iraq. A deputy foreign minister, Bassam Salih Kubba, was shot and killed Saturday and Kamal Jarrah, who directed cultural contacts with foreign countries in the Education Ministry, was gunned down Sunday.

In part because of the oil wealth that lies beneath its soil, Kirkuk has become a source of tension between the Kurds, who insist the city is part of their ancestral lands, and the Arabs and Turkmens who also inhabit the area and resent Kurdish authority. In addition, non-Kurdish interim government officials in Baghdad have made it clear that, over time, they intend to restore central authority to Kirkuk and the rest of Iraq's Kurdish-inhabited northern zone.

"We want to meet their demands for security but also achieve a balance between Kurds' desire to be a distinct component of Iraqi society and the interests of Iraq as a whole," Vice President Ibrahim Jafari of the Shiite Dawa party said in a recent interview.

In contrast to the sabotage and bloodshed elsewhere, the city of Najaf, about 90 miles south of Baghdad, got some good news. Moqtada Sadr, the Shiite Muslim cleric whose militia forces have been confronting U.S. occupation troops in the area since April, ordered his men to go home and leave the streets of the holy city to Iraqi police forces.

"To each and every member of the loyal army of the imam, who put on the line all that was dear and precious to them, and did not fail the Lord or their society, they are to return to their home provinces to carry out their duties and do what pleases God, his messenger and his household," read an announcement signed by Sadr.

The order was Sadr's latest gesture heeding the Shiite religious and political figures who have been urging him to lay down his arms and turn his Mahdi Army militia into a political movement. Sadr's spokesmen announced two days ago that Sadr intends to form a political party. Sadr's militia agreed more than a week ago to a cease-fire with U.S. forces in Najaf and the neighboring city of Kufa.

Although Wednesday's order fell short of disbanding the militia, it was a step in that direction and welcomed by those urging Sadr to turn to peaceful politics.

"I was very glad to hear that, and also what President Bush said," said Muhammed Bahr Uloum, who was oil minister under the now-disbanded Governing Council and has been involved in the efforts to draw Sadr into the political process.

Bush said on Tuesday in Washington that he would have no objection if the interim Iraqi government welcomed Sadr into the country's new political life and elections scheduled for January. His attitude contrasted with that of the top U.S. spokesman here, Dan Senor, who has repeatedly declared that the young cleric must answer a warrant charging him with involvement in the murder last year of a fellow cleric and, under a recent government order, cannot hold office for three years because he leads an illegal militia.

Najaf residents applauded the order for Sadr's nonresident fighters to return home. Many Najaf merchants have been hurt badly by the weeks of fighting and tension in their city, normally the destination of Shiite pilgrims from around the Middle East, particularly Iran, because of its sacred shrines.

"We thank Moqtada Sadr because he respected the will of the religious leadership and the will of the Shiite House," said Khalid Hussein, 31, a Najaf goldsmith, referring to the main organization of Shiite elders. "Life will go back to normal in the city. Iranians will come and visit the holy shrines again, and we'll get our business back. We also thank the new government, because they respect Moqtada Sadr."

Bahr Uloum said the sabotage on southern Iraq's pipelines -- the second such attack in as many days -- meant exports estimated at 1.1 million barrels a day would come to a halt until repairs are made. Although workers in the area have experience with making such repairs, he said the pipes must be drained before they can be fixed, meaning at least several days of work.

Exports through terminals around the port of Basra, at the head of the Persian Gulf, had been running at 1.65 million barrels a day, he added, but dropped by a third because earlier sabotage to a 48-inch pipeline left the Southern Oil Co. with only a 32-inch pipeline.

"It looks like they have now hit the two pipelines, which totally stopped production," Bahr Uloum said.

Northern production also has been squeezed by sabotage since an attack on May 25 interrupted normal use of the pipeline running from Kirkuk to the Turkish port of Ceyhan on the Mediterranean. Bahr Uloum said Iraqi oil workers were getting an average of 250,000 barrels a day to Ceyhan, sending it in intermittent flows through patches and letting it accumulate in storage tanks pending sale and lifting by tankers.

Mourners carry a portrait of Ghazi Talabani, the chief security official for Iraq's Northern Oil Co., along with his coffin, which is draped with a Kurdish flag.