BAGHDAD (Reuters) - A roadside bomb killed three U.S. soldiers in Iraq on Saturday, pushing the U.S. death toll closer to the 4,000 mark at the start of the sixth year of the war for U.S. troops.

The deaths, which brought the number of U.S. soldiers killed since the U.S.-led invasion in 2003 to 3,996, came three days after President George W. Bush said the United States was on track to victory in Iraq.

In an upbeat speech marking the fifth anniversary of the war, Bush acknowledged the “high cost in lives and treasure” but said a U.S. troop build-up in Iraq had reduced violence there and opened the door to a strategic victory in the war on terror.

The war is a major issue in the presidential campaign, with Democratic presidential candidates Barack Obama and Hillary Clinton calling for an early troop withdrawal timetable.

Presumptive Republican nominee John McCain wants to keep troops in Iraq until it is more stable.

The U.S. military said the three soldiers were killed when a roadside bomb blew up near their vehicle northwest of Baghdad. Two Iraqi civilians also died in the attack. It gave no further details about where the incident occurred.

Roadside bombs are the biggest killers of soldiers in Iraq.

On Friday, a U.S. soldier died from wounds sustained from “indirect fire”, a term commonly used by the military to refer to a mortar or rocket attack, south of Baghdad.

Six members of a U.S.-backed neighborhood patrol group were killed early on Saturday in a U.S. helicopter strike on their checkpoint in Salahuddin province, police and a local tribal leader said.

The U.S. military said it had conducted a helicopter attack in the province, but denied it had attacked a checkpoint. It said the strike killed six men suspected of placing roadside bombs. Investigations were under way, the military said.

SUNNI PATROL TENSIONS

The U.S. military has credited the formation of what it calls Concerned Local Citizen groups (CLCs), also known as Awakening Councils, for playing a crucial role in a 60 percent drop in violence across Iraq since last June.

The mostly Sunni Arab neighborhood patrols have some 90,000 men in western Anbar and provinces north and south of Baghdad. The U.S. military pays them $300 a month to patrol their neighborhoods and man checkpoints.

U.S. soldiers stand near the Swords of Qadisiyah monument in Baghdad March 13, 2008. REUTERS/Ceerwan Aziz

Tribal leader Abu Faruq said Saturday’s air strike took place on a CLC checkpoint near the town of Ishaqi, 100 km (60 miles) north of Baghdad.

“They knew all this area under is my control, and all the men were in uniform and were not firing their weapons, so why did this happen? If Awakening checkpoints are hit this way, it is a disaster,” he said.

The incident is the latest in a string of disputes between the CLCs and the U.S. military. In November, U.S. warplanes attacked a CLC checkpoint north of Baghdad, killing 25 men.

In February, CLCs in Jurf al-Sukr, south of Baghdad, said U.S. forces killed three of their number, and in the same month, neighborhood patrols in Diyala province, north of Baghdad, temporarily stopped working to demand more pay and the removal of a local police chief.

The southern Baghdad districts of Shurta and Hay al-Amil and the southern city of Kut were reported to be quiet on Saturday after Mehdi Army fighters loyal to Shi’ite cleric Moqtada al-Sadr clashed with Iraqi and U.S. forces a day earlier.

Sadr imposed a unilateral ceasefire on his unruly militia last August and extended it last month, a move U.S. commanders say has helped to reduce violence in Iraq.

But the gunbattles in Baghdad and Kut have raised fears that it may be unravelling at a time when the U.S. military is in the process of withdrawing 20,000 troops.

Mehdi Army fighters have complained that the truce ties their hands and opens them to attack by rival Shi’ite factions and U.S. forces. U.S. commanders say they only target Mehdi Army units that have ignored Sadr’s ceasefire order.