(adds another Sunni leader arrested, paragraph 7)

BAGHDAD, April 3 (Reuters) - U.S. forces said on Friday they opened fire on a group of fighters who could belong to a Sunni Arab patrol unit, days after the arrest of members of another unit by Iraqi forces triggered a gun battle.

The incident could further heighten tensions with the Sunni forces, who number some 90,000 and whom the U.S. military had backed to steer Iraq's Sunni Arabs away from an anti-U.S. insurgency.

The arrest of Adil al-Mashhadani, a Baghdad Sunni Arab force leader, started clashes last week between his supporters and Shi'ite-led government forces that killed three people.

The U.S. military said on Friday its planes had fired on four armed men seen planting a roadside bomb late on Thursday in the north Baghdad district of Taji, killing one of the men and wounding two others.

Initial investigations showed that at least one of the men was listed among the U.S.-backed Sunni fighters, who call themselves Awakening Councils -- "Majalis al-Sahwa" in Arabic -- but which the U.S. military calls "Sons of Iraq".

"Hostile acts will be engaged. While we value our Sons of Iraq brothers, these men had broken faith with their fellow Sons of Iraq, the Iraqi people and us," U.S. Major-General Daniel Bolger said in a statement.

In a separate incident, Iraqi police arrested Hussam Alwan, another Sahwa leader in the town of Moqtadiya, 80 km (50 miles) northeast of Baghdad, an Iraqi security source told Reuters. He declined to say for what charges Alwan was detained.

The Sahwa movement was started by Sunni Arab tribal sheikhs in western Anbar province who turned against al Qaeda in late 2006. The U.S. military backed them as a counter-insurgency tactic and the model was rolled out across Iraq.

Their programme has since last year been progressively handed over to the government, which has begun paying them.

The guards comprise many former insurgents, who are worried the government may arrest them. How Baghdad handles them is seen as a test of sectarian reconciliation in Iraq.

"It is a message to those taking the same path as (Mashhadani's) gang has taken that they will face the same destiny," Iraqi Prime Minister Nuri al-Maliki told Iraqi state TV on Thursday, referring to Mashhadani's arrest.

Mashhadani was arrested on suspicion of links to bomb making-cells, kidnappings, extortion and al Qaeda. (Reporting by Mohammed Abbas, Tim Cocks and Khalid al-Ansary)

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