Four bombs in mainly Shi'ite Muslim areas in Iraq's capital Baghdad killed at least 25 people and wounded dozens of others on Thursday, police and hospital sources said.

Fears are escalating of increased sectarian strife after Shi'ite Prime Minister Nuri al-Maliki sought the removal of two senior Sunni politicians just as U.S. troops withdrew from Iraq - the worst political crisis in a year.

In Baghdad's northwestern Kadhimiya district, two car bombs killed at least 15 people and wounded 32 others, the sources said.

Another two bombs, one planted on a parked motorcycle and another, also a roadside device, killed at least 10 people and wounded 37 others in the impoverished Sadr City district in northeastern Baghdad, they said.

There was a group of day labourers gathered, waiting to be hired for work. Someone brought his small motorcycle and parked it nearby. A few minutes later it blew up, killed some people, wounded others and burned some cars, said a police officer at the scene, declining to be named.

A Reuters reporter said there were blood stains all around the site of the motorcycle bomb attack and that tarmac on the road had been ripped up by the explosion. Building tools and shoes were scattered across the site.

Police said they found and defused two other bombs.

Iraq is still plagued by a deadly Sunni Muslim insurgency and Shi'ite militias nearly nine years after the U.S.-led invasion that ousted Saddam Hussein.

Sadr City is a stronghold of radical Shi'ite cleric Moqtada al-Sadr, whose Mehdi militia once fought U.S. and Iraqi troops. He is now a key ally of Maliki.

The premier angered rivals when he asked parliament to have his Sunni deputy Saleh al-Mutlaq removed and sought an arrest warrant for Iraq's Sunni Vice President Tareq al-Hashemi on charges he ran death squads.

On Tuesday, members of the Sunni-backed Iraqiya bloc boycotted Iraq's parliament and cabinet, accusing Maliki's bloc of governing alone in a power-sharing coalition that was supposed to ease sectarian tensions.

A spate of bombings that killed 72 people in mainly Shi'ite areas of Baghdad a few days after the political crisis began has only deepened the fears of a return to sectarian strife in Iraq, which teetered on the brink of civil war in 2006-7.

The inclusion of Iraqiya in the governing coalition was widely considered crucial to prevent a return to the kind of sectarian violence that was unleashed after the 2003 U.S.-led invasion. Thousands were killed in the violence.

Many Sunnis complain of having been sidelined in the political process since Saddam was ousted and the majority Shi'ites dominated the government.

(Writing by Serena Chaudhry; Editing by Louise Ireland)