A suicide bomber exploded a white Oldsmobile outside a police station in this northern city Monday, killing at least seven policemen and wounding as many as 52 other people. It was the fifth homicide attack in Iraq this month.

The bombing occurred as Defense Secretary Donald H. Rumsfeld (search) visited Baghdad to check on the state of readiness of Iraq's security forces, which have born the brunt of the strikes.

U.S. administrator L. Paul Bremer (search) told reporters after meeting with Rumsfeld that Iraq has seen "a real step up" by "professional terrorists from Al Qaeda and Ansar al-Islam (search) in conducting suicide attacks."

Kirkuk has also seen rising ethnic tensions as Kurds, Arabs and Turkomen compete for control of the city, located in one of the world's richest oil-producing regions, 180 miles north of Baghdad.

The bomber detonated his 1990 car as police were changing shifts at Rahimawa station, according to the station's chief, Col. Adel Ibrahim.

Col. Thamer Abdul-Masih said the bomber's car followed policemen driving to the station in a Kurdish neighborhood and "ran into the last car in the convoy and exploded." U.S. command in Baghdad said Iraqi police fired on the car but were unable to stop it.

Police Chief Torhan Yousef said seven policemen and the bomber were killed and 52 people were wounded. The U.S. military said 35 people were wounded.

The blast devastated nearby buildings and injured civilians in a passing bus.

"I fell on the floor of the bus," said Awen Aras, 11, as she lay in a hospital bed, her leg in a cast. "Everything was flying around me after I heard a very loud explosion. There was a big fire and policemen carried me off the bus."

More than 300 people — mostly Iraqis — have been killed in bombings against Iraqi security forces this year. Just this month, bombers have struck Kurdish political offices in Irbil, a police station in Iskandariyah, an army recruiting station in Baghdad and a Polish-run military garrison in Hillah.

The United States intends to transfer power to the Iraqis on June 30, but its plans have caused controversies on several fronts. Leaders of the country's Shiite Muslim majority, long repressed under deposed dictator Saddam Hussein, demand quick elections to establish a government.

The United Nations backed the U.S. stance that elections are impossible by June 30 but said Monday that a vote could be held by January 2005 if planning begins at once. The report by U.N. envoy Lakhdar Brahimi also said tensions between Sunni and Shiite Muslims were "becoming entrenched and inter-communal politics more polarized."

Bremer welcomed the U.N. report, calling it a "constructive contribution.

"We share the U.N.'s view on the importance of direct elections in Iraq as soon as possible," Bremer said. "The report makes clear that we must stand firm in handing sovereignty to the Iraqi people on June 30, and we will meet that milestone."

White House spokesman Scott McClellan said the administration favors "moving forward as quickly as possible on elections."

U.S. officials also have warned of growing violence as the deadline for the transfer of power approaches.

The confusion over the transfer plan will also delay an agreement between the Iraqis and Americans on the status of U.S. military forces after the return of sovereignty, Iraqi officials said Monday.

The status of forces agreement was due to be reached by the end of March.

Governing Council member Adnan Pachachi said the agreement would wait until a provisional government takes power because the council "is not considered sufficiently representative."

Washington had planned to establish a provisional legislature through regional caucuses. The legislature would have then appointed a government to take power June 30 and rule until elections in 2005.

Those plans were thrown into disarray when Shiite clergy demanded that voters choose the legislature in a direct national election. Now that the caucus plan is dead, the United States would prefer handing power to an expanded Iraqi Governing Council.

A senior Iraqi cleric said Monday that delaying national elections would be a "time bomb that could explode at any minute."

"Without elections, our national institutions will remain shaken, unrecognized and distrusted by the people," Grand Ayatollah Mohammed Taqi al-Modaresi told reporters in Karbala. The uncertainty "makes us fear for the future of Iraq" and the possibility of "civil war," he said.