(Adds more Bush quotes)

By Paul Tait and Mohammad Zargham

BAGHDAD/WASHINGTON, Aug 22 (Reuters) - Iraqi Prime Minister Nuri al-Maliki reacted sharply on Wednesday to U.S. criticism of his government's slow progress toward reconciliation, and U.S. President George W. Bush restated his support for Maliki after earlier lukewarm comments.

Bush also drew a comparison with the U.S. experience in Vietnam as he urged perseverance in the unpopular Iraq war. He cited the example of the emergence of the Khmer Rouge in Cambodia and violence in Vietnam after U.S. troops pulled out to warn of the consequences of leaving Iraq.

The comparison with Vietnam is one the Bush administration has tended to avoid. Many Democrats have likened Iraq to Vietnam, calling the war a quagmire that has exacted a toll in American lives and money without furthering U.S. interests.

U.S. officials have voiced increasing frustration this week with Maliki's failure to advance political reform despite an increase in the number of U.S. troops, which is intended to give breathing room to his fractured Shi'ite-led coalition.

On Wednesday, Hillary Clinton, the leading Democratic candidate in the 2008 presidential race, joined another Democratic senator's call for Maliki to be replaced.

Even Bush, speaking to reporters in Canada on Tuesday, noted "a certain level of frustration with the leadership" and failed to offer a direct endorsement of Maliki. Earlier on Tuesday, U.S. Ambassador to Iraq Ryan Crocker said the Maliki government's progress toward reconciliation was "extremely disappointing."

Maliki hit back during a visit to Damascus, saying no one outside Iraq had the right to set timetables for progress.

On Wednesday, Bush said in a speech to U.S. war veterans in Kansas City, Missouri: "Prime Minister Maliki is a good guy, a good man, with a difficult job and I support him."

As the diplomatic heat rose, there was more violence in Iraq. A suicide bomber killed 25 people, including 15 policemen, and wounded 73 in an attack on a police headquarters in Baiji. Fourteen U.S. soldiers were killed in a helicopter crash in northern Iraq blamed on mechanical failure, the worst incident of its kind since January 2005.

Bush's Republican administration is pushing for Iraq's warring Shi'ite Muslim majority and minority Sunni Muslim Arabs to reconcile so it can start bringing troops home from an unpopular war. Crocker and the top U.S. commander in Iraq, Gen. David Petraeus, are due to deliver a progress report next month that could signal a change in U.S. policy in Iraq.

Maliki's response to the criticism was blunt. "The Iraqi government was elected by the Iraqi people," he told reporters in the Syrian capital.

"Maybe this person who made a statement yesterday is upset by the nature of our visit to Syria," Maliki said, without making clear if he was referring to Bush or Crocker.

"These statements do not concern us a lot," Maliki said. "We will find many around the world who will support us in our endeavor."

'IDEOLOGICAL STRUGGLE'

In his Kansas City speech, Bush said the war in Iraq, like World War Two, the Korean War and the Vietnam War, was an "ideological struggle," and he again depicted the conflict as part of the broader U.S. war on terrorism.

"The militarists of Japan and the communists in Korea and Vietnam were driven by a merciless vision for the proper ordering of humanity," Bush said.

"Like our enemies in the past, the terrorists who wage war in Iraq and Afghanistan and other places seek to spread a political vision of their own: a harsh plan for life that crushes freedom, tolerance and dissent," he added.

The U.S. military has launched a nationwide offensive targeting Sunni Islamist al Qaeda fighters and Shi'ite militias to thwart an expected increase in attacks ahead of the report to Congress, expected on Sept. 11 or 12.

Washington has set a series of benchmarks for Maliki's government, including a revenue-sharing oil law, which it sees as key to ending the sectarian conflict that has killed tens of thousands of Iraqis.

Reporting the helicopter crash, a U.S. military statement said initial indications suggested the Black Hawk, one of two involved in night operations, had suffered mechanical failure. It was the second incident of its kind in eight days.

"There were no indications of hostile fire," it said.

"That helicopter had been carrying four crew members and 10 passengers." The exact location of the crash was not immediately clear.

The deaths took to 3,721 the number of U.S. military killed in Iraq since the 2003 invasion to topple Saddam Hussein, including 63 this month. (Additional reporting by Damascus bureau and Aseel Kami, Ross Colvin in Baghdad, and Caren Bohan in Kansas City)

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