BAGHDAD — Sunni extremists fired mortars into a shrine in Samarra that is sacred to Shiites late Monday, killing six people and damaging the shrine, according to a report on Iraqi state television.

Attacks on Samarra’s Askariya Shrine by Qaeda-related groups in 2006 set off a wave of sectarian violence throughout Iraq that took years to calm.

A report on the Iraqiya television network said four mortar shells were fired into the compound of the mosque, which was built in 944 and is considered one of the most important shrines in the Shiite world.