A series of attacks in Iraq on Sunday killed at least six people and wounded 29, part of a major upswing in violence after months of calm.

Police officials said four bombs planted in cars went off around Baghdad and another north of Baghdad. Authorities said the bombs mainly targeted Shiite pilgrims making their way to the holy city of Karbala for the annual Arbaeen ritual, which marks the end of the 40-day mourning period for Imam Hussein, a central figure in Shiite Islam.

The latest attacks come at the end of particularly violent week in the country. Suicide bombers targeting police, police recruits and Shiite pilgrims have already killed more than 100 people and injured hundreds more.

The three-hour drumbeat of explosions began around 7 a.m. in Baghdad’s rush hour at the start of the local work week. No group immediately took responsibility for the bombings, but a senior Iraqi intelligence official said they appeared to be the work of insurgents taking advantage of the government’s delay in appointing a new interior minister, who runs the nation’s security forces.

Prime Minister Nouri al-Maliki is still weighing who to name to the nation’s top defense, interior and national security posts, saying he wants to ensure they are filled by apolitical candidates.

The intelligence official said the attacks were trying to present the country as dangerous because Arab heads of state and their large retinues plan to meet in Baghdad in late March for the annual Arab League summit, the first time the city hosts the summit in 20 years.

Violence in Iraq has dropped dramatically since just a few years ago when daily death tolls sometimes were in the hundreds from sectarian killings. But small scale bombings and drive-by shootings still persist on a near daily basis.

NPR’s Kelly McEvers contributed to this report, which includes material from The Associated Press Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.