By Chris Collins and Leila Fadel | McClatchy Newspapers

BAGHDAD — A roadside bomb Saturday killed the governor and police chief of an oil-rich southern Iraqi province that's been a bloody and complex battleground for U.S.-Iraqi security forces and powerful Shiite factions.

Elsewhere, Gen. David Petraeus, the top U.S. military commander in Iraq, added his voice to U.S. claims linking Iran to the deaths of increasing numbers of American troops.

Meanwhile, on a third front, a U.S. commander working with Iraqi troops to pacify Diyala province said things were going surprisingly well.

Killed in the roadside blast were Khalil Jalil Hamza, governor of Diwaniyah province, whose capital lies about 100 miles south of Baghdad, and the provincial police chief, Maj. Gen. Khalid Hassan.

Hamza was a member of the influential Supreme Islamic Iraqi Council, headed by Shiite politician Abdul Aziz al Hakim. He was also a senior member of the council's militant wing, the Badr Organization. The Badr militia is locked in combat in Diwaniyah and elsewhere with the Mahdi Army, which rival Shiite cleric Muqtada Sadr loosely controls.

Hoping to avert a factional flare-up, police immediately ordered a curfew in Diwaniyah and a ban on vehicular traffic. Prime Minister Nouri al Maliki issued a statement pleading for calm. He promised an inquiry that would end, he pledged, in the "arrest of the people who did the terrible crime."

Gen. Petraeus, as he toured a Joint Security Station near the town of Hussainiyah, north of Baghdad, told reporters that an increase in deadly explosively formed projectiles — which are more powerful than familiar improvised explosive devices — showed Iran's hand.

"The number of EFP attacks has increased, so that is clearly a function of the Iranian support," Petraeus said.

As evidence, he cited a taped interrogation of Qais Khazaali, a former Sadr aide who was detained in Basra earlier this year for running an EFP supply line.

During the interrogation, Petraeus said, Khazaali was asked whether he could conduct operations without Iran's help. In response, Khazaali threw back his hands and laughed, saying, "Of course not."

Petraeus said the tape, which has not been made public, had been shown to Iraqi officials.

U.S. officials have repeatedly accused Iran of arming and supporting factions of the Mahdi Army. The army's been accused of wide-scale sectarian killings as well as intense battles with other Shiite factions in the south.

Last week, Lt. Gen. Raymond Odierno, the No. 2 U.S. military commander in Iraq, said that Shiite militiamen, whom the U.S. says Iran supports, had launched 73 percent of the attacks that had killed or wounded American troops in July.

On Thursday President Bush warned Iran of unspecified consequences if it continued to arm and train insurgents attacking coalition forces.

Also Saturday, the U.S. commander of efforts to transform Diyala province northeast of Baghdad into a region secured primarily by Iraqi forces declared his mission a success so far.

In a video conference with reporters, Lt. Col. Andrew Poppas said U.S.-led coalition forces have "deliberately and methodically freed villages from the yoke of al Qaeda oppression," referring to the Sunni insurgent group al Qaeda in Iraq.

He said that many routine patrols in the Diyala River Valley, which includes the city of Baqouba, are now led by Iraqi military units. In some villages, Poppas said, Iraqi soldiers and police "solely" maintain security.

The commander attributed the success of his operation, which began in March, to a military strategy that immerses troops within villages and cities, allowing them to work with locals on a daily basis. He said those tactics have yielded invaluable intelligence that allowed military forces to weed out troublemakers.

Special correspondent Hussein Kadhim contributed to this report.