U.S. troop reinforcements began patrolling the streets of Baghdad Sunday as part of a major effort to reduce deadly sectarian violence in the Iraqi capital.

About 3,700 U.S. soldiers from the army's 172nd Stryker Brigade have been sent to Baghdad after completing a mission in northern Iraq.

The U.S. reinforcements have been deployed to some of the most violent, Sunni-dominated neigbhborhoods in western Baghdad.

Iraq has been engulfed in a wave of sectarian violence between Shi'ite and Sunni Arabs since the February bombing of a Shi'ite mosque in Samarra.

The U.S. military's 172nd Stryker Brigade had been due to return home after a 12-month tour of duty. The extended deployment of the brigade means the overall U.S. force in Iraq remains at around 130,000.

Iraqi officials say at least 21 people were killed Saturday in several attacks across the country.

Authorities also say more than more than 180-thousand people have been displaced by sectarian strife over the past six months.

Some information for this report was provided by AFP, AP.