Story highlights The bomber targeted a Shiite mosque in northeast Baghdad

ISIS has claimed responsibility for some recent attacks in Baghdad

A suicide bombing in a predominately Shia area of Baghdad has left 21 people dead and 25 people injured, according to a police source with knowledge of the incident speaking to CNN by phone. The source asked not to be identified for security reasons.

The bomber targeted a Shia mosque in the city's northeast al Harithyia neighborhood, the source said.

As of Sunday, there has been no claim of responsibility.

The past few weeks have seen a series of car and suicide attacks on Shia targets, some of which have been claimed by ISIS.

Although Baghdad's perimeter has appeared to hold firm against ISIS encroachment, the militant Sunni group has made strategically valuable gains in the country's Anbar province, west of the capital.

ISIS, which calls itself the Islamic State, has been accused of massacring Shiites in areas it controls.