(Adds reaction by Iraqi prime minister, U.S. officials)

By Paul Tait

BAGHDAD, Aug 11 (Reuters) - The governor and police chief of Iraq's Shi'ite province of Diwaniya were killed when a roadside bomb hit their convoy of vehicles on Saturday, police said.

Diwaniya governor Khalil Jalil Hamza and police chief Major-General Khaled Hassan were returning to the provincial capital of the same name, 180 km (110 miles) south of Baghdad, when their convoy of four-wheel drives was hit.

Hassan had been in the job for less than a week, police said. Hamza was a member of the Supreme Islamic Iraqi Council (SIIC), the biggest Shi'ite party in Iraq.

They had been attending the funeral of a leading tribal sheikh in the town of Efaj, 30 km (18 miles) east of the city.

Three of their bodyguards were killed and three others wounded. Local authorities quickly imposed an indefinite curfew.

One policeman said the bomb was an "explosively formed penetrator", a particularly deadly form of armour-piercing bomb which U.S. forces accuse Iran of supplying to Iraqi militias.

The attack was condemned by Iraqi Prime Minister Nuri al-Maliki, who called for calm after a "seditious act" designed to create instability, and by U.S. Ambassador Ryan Crocker and U.S. military commander General David Petraeus.

"Building unity amid violence requires special commitment and courage. These two men were examples of the courage of the Iraqi people in the face of barbaric extremists," Crocker and Petraeus said in a joint statement.

CONFLICT

The SIIC's armed wing, the Badr Organisation, has been in conflict with the Mehdi Army militia of powerful anti-American Shi'ite cleric Moqtada al-Sadr.

The Shi'ite-dominated south has become increasingly restless as factions vie for control of the oil-producing region, often pitting police loyal to one bloc against militiamen of others.

U.S. forces have also been pursuing what they describe as rogue elements of Sadr's Mehdi Army, who are accused of bringing in weapons and receiving training from neighbouring Iran. There were heavy clashes in mid-April when U.S. and Iraqi forces fought to wrest control of the city back from the Mehdi Army.

The second most senior U.S. general in Iraq said Iran was supplying militias in Iraq with more weapons to attack U.S. troops in a bid to influence debate on the war in Washington ahead of a crucial progress report due next month.

"In the last three months ... we are seeing brand-new rocket launchers, mortars and mortar launchers," Lieutenant-General Raymond Odierno told Reuters in an interview.

Iran denies meddling in Iraq and blames the U.S.-led invasion in 2003 for triggering Iraq's sectarian strife.

North of Baghdad, U.S. forces have claimed success in denying Sunni Islamist al Qaeda control of towns and villages along the fertile Diyala River valley in Diyala province.

Thousands of extra U.S. and Iraqi forces were sent to religiously mixed Diyala in recent months after a security crackdown launched in Baghdad in mid-February forced al Qaeda fighters and insurgents out of Baghdad into surrounding areas.

"We have forces throughout the Diyala valley in key critical nodes. We cross any line of communications, deny the enemy freedom of movement. Everything they do is watched," Lieutenant-Colonel Andrew Poppas, commander of U.S. forces in the area, told reporters in Baghdad by video link.

The security operations in Baghdad and Diyala are part of a wider push utilising 30,000 extra U.S. troops meant to buy time for Maliki's fractured coalition government to reach political targets set by Washington.

The benchmarks, including a revenue-sharing oil law, are designed to promote reconciliation between Iraq's warring Shi'ite majority and minority Sunni Arabs who were dominant under Saddam Hussein.

Washington is unhappy with Maliki's slow progress towards the benchmarks and U.S. President George W. Bush is under increasing pressure to show results in the unpopular war or start bringing troops home.

Crocker and Petraeus are due to report to Congress in mid-September on political and security progress in Iraq. (Additional reporting by Ross Colvin and Peter Graff in Baghdad)

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