BAGHDAD, May 6 (Reuters) - The third of five U.S. extra brigades sent to Iraq last year to help curb spiralling sectarian violence is returning home, the U.S. military said.

The redeployment of about 3,500 soldiers from the 3rd Brigade, 3rd Infantry Division in the next several weeks, is part of a wider plan to withdraw the five "surge brigades", or 20,000 troops, by the end of July.

U.S. President George W. Bush last month suspended further troop withdrawals from July, imposing a 45-day freeze on the remaining 140,000 troops to assess the security situation before considering more cuts.

The brigades are credited with helping to reduce violence in Iraq by 60 percent since last June, but the U.S. commander in Iraq, General David Petraeus, told U.S. Congressional hearings in April that progress was "fragile and reversible".

"The continuing drawdown of surge brigades demonstrates continued progress in Iraq," Brigadier-General Dan Allyn, chief of staff of Multi-National Corps - Iraq said in a statement late on Monday.

But the drawdown comes at a time when U.S. forces are locked in daily street battles in Baghdad with Shi'ite militia fighters loyal to anti-American cleric Moqtada al-Sadr.

The fighting has taken a deadly toll on U.S. soldiers, threatened to unravel many of the security gains and drawn U.S. forces deeper into the conflict between the Shi'ite-led government and Sadr, a populist Shi'ite leader.

The five brigades were sent to Iraq to help give Iraqi political leaders time to reach a political accommodation and defuse sectarian tensions. Critics say the government has largely failed to take advantage of this window of opportunity.

The Iraq war is a major issue in the November U.S. presidential election, with Democratic nominees Barack Obama and Hillary Clinton calling for troops to be brought home as soon as possible and Republican candidate John McCain warning of the dangers of withdrawing the troops too soon.

The 3rd Heavy Brigade Combat Team, from Fort Benning, Georgia, has been deployed in Baghdad province since March 2007 and was charged with stemming the flow of materials for bombs into the capital.

The U.S. military said in a statement that the Combat Team had captured more than 600 suspected militants, unearthed many weapons caches and significantly reduced violence in their area. (Writing by Ross Colvin; Editing by Charles Dick)

AlertNet news is provided by