(Corrects location of attack to northern Baghdad paragraph 4)

By Peter Graff

BAGHDAD, Dec 15 (Reuters) - Gunmen and bombers launched three attacks on U.S.-backed neighbourhood security patrols in Baghdad on Saturday, killing at least three of the patrol members and wounding 17.

U.S. forces in Iraq have increasingly relied on neighbourhood patrols to keep peace in mainly Sunni Arab parts of Iraq as part of a strategy that has helped bring violence levels down dramatically over the past several months.

But the patrol members, who are paid by U.S. forces and not officially part of the Iraqi security forces, have increasingly come under attack by militants.

In one incident on Saturday, bombers killed two patrol members and wounded 10 in a strike on their headquarters in the Adhamiya neighbourhood of northern Baghdad, until recently a Sunni Arab militant stronghold.

Gunmen attacked a patrol in another northern area, killing one patrol member and wounding four. In the southern Doura neighbourhood, another former Sunni militant stronghold, gunmen wounded three patrol members manning a checkpoint.

U.S. forces are trying to isolate al Qaeda fighters by recruiting Sunni Arabs who have turned against the radical Sunni Arab militants, and by launching regular offensives with Iraqi forces against their hideouts.

The latest offensive started early on Saturday in the Babel province south of Baghdad. It involves Iraqi army soldiers and U.S. troops from the 4th Brigade Combat Team, part of the 3rd Infantry Division, the U.S. military said.

Operation Marne Roundup is focused on "flushing out al Qaeda extremists and weapons smugglers operating" near the town of Iskandariya, 40 km (25 miles) south of Baghdad, the military said in a statement.

U.S. commanders said last week the assault would involve around 1,400 U.S. troops and will target Sunni Islamist fighters in small hamlets and fishing villages along the Euphrates River valley in Babel province.

By Saturday afternoon, the troops had faced no resistance as they moved to the target area, the statement said.

PATROL UNITS

Suspected al Qaeda fighters have attacked both Shi'ites and Sunnis in Babel. In one incident more than a month ago, a suicide bomber killed a prominent Sunni tribal leader involved in establishing neighbourhood patrol units in north Babel.

U.S. commanders credit the neighbourhood police units as being one of the main reasons why the number of attacks across Iraq has fallen 60 percent since June.

U.S. officials call the patrols "concerned local citizens" and pay some 50,000 patrol members about $10 a day. They are expected to provide their own weapons but are issued ID cards and simple uniforms such as reflector vests or shoulder belts.

Washington acknowledges some of the patrol members may have had links to insurgent groups but says they are screened to weed out those responsible for attacks.

The Shi'ite-led government was initially lukewarm over the prospect of men it once regarded as enemies being permitted to take up arms, but now says it will take over the programme from the U.S. military and put most of the patrols on its payroll. (Additional reporting by Alaa Shahine and Aws Qusay; editing by Robert Woodward)

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