(Adds Pace, Rice comments, Turkey troop build-up)

By Alister Bull

BAGHDAD, July 13 (Reuters) - U.S. soldiers killed at least 13 people including six Iraqi policemen after coming under fire from a police checkpoint in Baghdad on Friday, the U.S. military said.

An Iraqi reporter working for The New York Times was also shot dead on his way to work in Baghdad, the newspaper said, a day after two Reuters employees were killed during an incident involving U.S. forces in the city.

Khalid Hassan, 23, was shot in the southern Saidiya district of the capital, the Times said in a statement, adding that the circumstances of the attack were unclear.

A U.S. warplane made a strike during the raid in mainly Shi'ite east Baghdad after U.S. soldiers came under "heavy and accurate" gunfire from a police checkpoint, rooftops and a church, the military said in a statement.

Seven suspected militants were also killed during the clash, in which U.S. soldiers detained an Iraqi police lieutenant on suspicion of planning roadside bomb and mortar attacks on U.S. forces. The military accused him of links with Iranians accused by Washington of fomenting violence in Iraq.

"When they (U.S. forces) went to arrest this lieutenant, some of the police who were with him began firing on our folks," Marine Gen. Peter Pace, chairman of U.S. military's Joint Chiefs of Staff, told reporters in Washington.

"That turned those individuals into enemy and legitimate folks for our troops to take on in combat."

U.S. commanders say elements of the police are infiltrated by sectarian militia, making it harder to restore security and ease tension between majority Shi'ite and minority Sunni Arabs.

Pace also said that the number of Iraqi army battalions that operate independently, with no assistance from U.S. forces, had dropped from 10 to six over the last two months, although he downplayed the change as partly due to temporary factors.

The relentless violence is raising pressure on U.S. President George W. Bush to start withdrawing U.S. troops, less than a month after all of an additional 28,000 troops arrived as part of a new attempt to boost security.

The Democrat-controlled U.S. House of Representatives voted for a withdrawal of combat troops by April 1, 2008, on Thursday in a largely symbolic gesture. Bush has vetoed previous congressional efforts to place a timetable on the war.

TURKEY TROOP BUILD-UP

U.S. Secretary of State Condoleezza Rice defended Iraq's mixed report in an interim report released this week, saying in a series of television interviews the Bush administration needs two more months to find a "coherent way forward."

"Right now we are still in the midst of the new strategy," Rice told ABC's "Good Morning America."

Shrugging aside critics, Bush said on Thursday he would wait for a September report from his top commander in Iraq, General David Petraeus, and Ambassador Ryan Crocker.

U.S. and Iraqi forces have launched a major clampdown in and around Baghdad to clear neighbourhoods of militant strongholds, and break up the networks supporting car bombers.

The military said it had detained more than 130 suspects in the southern Baghdad district of Rashid since July 1, including 31 at what it said might have been an al Qaeda meeting.

Iraq's northern neighbour Turkey has boosted troops levels along the border to more than 200,000, security sources told Reuters, raising fears of a wider conflict in a relatively peaceful region of Iraq.

NATO-member Turkey has refused to rule out a cross-border operation to crush up to 4,000 Kurdistan Workers Party rebels believed to be based in mountains in northern Iraq. Iraq has said its security forces are so thinly stretched that it does not have spare troops to send to the border region.

Iraqi police blamed U.S. military action for the deaths of Reuters photographer Namir Noor-Eldeen, 22 and driver Saeed Chmagh, 40, in Baghdad on Thursday. The U.S. military said the pair died after a clash between its troops and insurgents, adding that the incident was under investigation.

Paris-based press freedom advocacy group Reporters Without Borders said the contradictory statements demanded an investigation by both the U.S. military and Iraqi police.

Reporters Without Borders also urged the Iraqi authorities to establish the exact circumstances of Hassan's slaying.

The New York-based Committee to Protect Journalists says at least 149 reporters and media assistants have been killed since the U.S.-led invasion in 2003, the vast majority Iraqis, making it the most dangerous place in the world to report. (Additional reporting by Waleed Ibrahim, Mussab al-Khairalla in Baghdad; Kristin Roberts in Washington)

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