BAGHDAD — At least 15 people were killed in a series of bomb blasts across Iraq on Wednesday, police and medics said, following a sharp increase in violence that has prompted warnings of a full-blown sectarian conflict.

Violence in Iraq has increased as the civil war in neighboring Syria puts a strain on fragile relations between Sunni and Shiite Muslims. Tensions are at their highest in Iraq since U.S. troops pulled out more than a year ago.

A suicide bomber wearing an explosives vest detonated himself in the midst of a group of government-backed Sunni fighters who were collecting their salaries east of the city of Fallujah, killing six, police sources said.

In Baiji, 180 kilometers (112 miles) north of Baghdad, police said a roadside bomb killed four policemen.

A car bomb in a Shiite district in northeastern Baghdad killed at least three people and wounded 14, police and hospital sources said. Another car bomb north of the city of Ramadi killed two policemen and wounded another 10.

Iraq is home to a number of Sunni Islamist insurgent groups including a local al-Qaida affiliate that has launched frequent attacks to undermine the Shiite-led government's and provoke wider confrontation.

Violence is still well below its height in 2006-07, but provisional figures from rights group Iraq Body Count indicate the number of violent deaths in April was the highest monthly toll since 2009.