U.S.-led forces arrested 25 people during operations against Al Qaeda in Iraq this weekend, the American military said Sunday. Homicide bombers struck police targets in northern Iraq, killing five people and injuring dozens in the latest sign that extremists, though weakened, are still capable of devastating attacks.

The U.S. military said those detained included alleged members of car bomb cells and financial networks, and a suspect who allegedly assisted in the movement of homicide bombers from Iran to Iraq. Most of the suspects were detained in Baghdad and the northern cities of Mosul and Kirkuk, which remain volatile despite improvements in security in many parts of the country.

The U.S. military believes Al Qaeda in Iraq, a Sunni extremist group, has received financial aid and weapons from sources in Shiite-dominated Iran, even though Sunni extremists view Shiites as renegade Muslims and much of Iraq's violence has been sectarian.

U.S. officials have accused Tehran of supporting Shiite militias in Iraq as part of its power struggle with the United States for influence in the Middle East, though militia activity has dropped sharply. Some observers believe there have been links between Iran and Sunni extremists seeking a common goal of undermining the United States despite their traditional animosity.

"Coalition forces will continue to target the Al Qaeda network to further diminish its ability to conduct terrorist attacks against the Iraqi people," said Maj. John C. Hall, a U.S. military spokesman.

One homicide attack occurred in Kirkuk, a northern city with a volatile population of Kurds, Arabs and Turkomen whose disputed status has prevented parliament from passing a provincial election law that is key to reconciliation in Iraq. Kurds want Kirkuk incorporated into their self-ruled northern region, but Arabs and Turkomen want it to stay under central government control.

A policeman was among three people killed in the truck bombing at a police checkpoint, and 15 policemen were among the 23 injured, said Brig. Gen. Sarhat Qadir, head of Kirkuk's police. The attack happened in a Sunni Arab area.

Another homicide truck bombing hit a police headquarters in the city of Mosul, killing a policeman and a civilian, and injuring 40, many of them police. The building sits in a residential area and many houses were damaged, said an official in the operations command of Ninevah province who requested anonymity because he is not authorized to speak to the media.

Also in Mosul, gunmen killed two brothers and injured a third, police and hospital officials said. An Iraqi soldier was shot dead in a separate incident.

Gunmen in western Baghdad injured Brig. Gen. Adel Abbas, an Interior Ministry official, as he drove to work Sunday morning, two high-ranking Interior Ministry officials and the U.S. military said. Police and hospital officials had earlier said Abbas had been killed, and they spoke on condition of anonymity because they were not authorized to give information to the media.

But Maj. Gen. Hussein Ali Kamal, a deputy minister for intelligence affairs, and the ministry spokesman, Maj. Gen. Abdul-Karim Khalaf, said those reports were false.

"There were no deaths involved in this event," the U.S. military said in a statement. It said a bullet had grazed Abbas' arm, and that a security guard with Abbas was also shot.

South of Baghdad, officials in the Shiite holy city of Najaf stepped up security measures ahead of Monday's anniversary of the death of Imam Ali, the seventh century cousin of the Prophet Muhammad and the founder of the Shiite faith.

About 15,000 Iraqi policemen and soldiers were deploying in and around the city to protect hundreds of thousands of Shiite pilgrims, said Col. Ali Nomas, the Iraqi army spokesman. Insurgents have targeted Shiite pilgrims in the past.