BAGHDAD — Iraq’s defense minister promised Sunday to wage a new crackdown in a volatile province northeast of Baghdad where militants are trying to regroup after being routed from their urban stronghold there last summer.

Suicide attacks have killed more than 20 people in the last three days in Diyala province, a tribal patchwork of Sunni Arabs, Shiites and Kurds that stretches from Baghdad to the border with Iran.

Defense Minister Abdul-Qader al-Obeidi said preparations had begun for a fresh military operation in the provincial capital, Baqouba, about 35 miles from Baghdad.

“If we succeed in controlling areas of Diyala close to Baghdad, the rate of incidents in Baghdad decreases by 95 percent,” al-Obeidi said.

Violence has declined sharply in Iraq since June, when the influx of U.S. troops to the capital and its surrounding areas began to gain momentum. Also credited with the decline were the freeze in activities by the Mahdi Army militia, led by anti-American Shiite cleric Muqtada al-Sadr, and the decision by tens of thousands of Iraqis — most of them Sunni Arab — to join the fight against al-Qaida.

But it has been a constant challenge to subdue extremists in Diyala, which is the eastern gateway to Baghdad. More than two years ago, U.S. forces thought they’d turned the corner and American commanders handed over substantial control of the province to the Iraqi army in August 2005.

Al-Qaida began moving into Diyala in 2006 after losing its sanctuaries in Anbar province and declared Baqouba as the capital of the Islamic State of Iraq.

Last summer, American troops regained control of Baqouba in a pair of operations, restoring some government services and commerce.

But U.S. officers said at the time they expected the extremists to scatter to hills north of Baqouba and to the city of Muqdadiyah to the east and try to regroup.

Also Sunday, about the same time he was being praised by U.S. commanders Sunday for his role in improving security in Babil province south of Baghdad, a popular police chief was killed by a roadside bomb that struck his convoy. Major Gen. Qais al-Maamouri died along with two of his bodyguards in the explosion that occurred in Hillah, about 60 miles south of Baghdad, Iraq law-enforcement officials said.

At a lunchtime media briefing in Baghdad, U.S. military officials had praised al-Maamouri for his role in making the region more secure. “We’re very lucky in Babil province to have Major General Qais, who is a very good Iraqi police chief for that province,” Col. Tom James, commander of the 4th Brigade Combat Team of the U.S. Army’s 3rd Infantry Division, in north Babil, told reporters. “He is committed to securing Iraq for the people, the population. He does not see anything through a sectarian lens, it’s all about Iraqi law, and the people see that.”

In Basra, the police chief on Sunday said religious vigilantes have killed at least 40 women this year because of how they dressed, their mutilated bodies found with notes warning against “violating Islamic teachings,” the police chief said Sunday.

Maj. Gen. Jalil Khalaf blamed sectarian groups that he said were trying to impose a strict interpretation of Islam. They dispatch patrols of motorbikes or unlicensed cars with tinted windows to accost women not wearing traditional dress and head scarves, he added.