Suspected Shi'ite militiamen attacked Sunni Arab mosques in Iraq's capital Friday, in apparent retaliation for a wave bombings a day earlier that killed more than 200 people.
Witnesses say militiamen fired rocket-propelled grenades and machine guns, and set fire to four mosques during a rampage aimed at Sunni Arabs. Authorities say some 30 people were killed.
Meanwhile, witnesses say a U.S. helicopter exchanged fire with militiamen on the ground in Baghdad's Shi'ite stronghold of Sadr City, the scene of Thursday's deadly bombings.
The U.S. military says the helicopter destroyed rocket launchers after observing rocket fire coming from Sadr City.
There are reports of casualties.
In Washington, the Bush administration called Thursday's attack a senseless act. It was the deadliest since the Iraq war began in 2003.
U.N. Secretary General Kofi Annan condemned the violence and called for restraint from all Iraqis to prevent a further escalation.
In other bloodshed Friday, a suicide bomber killed at least 22 people in the northern city of Tal Afar. And British authorities say a British soldier died of wounds suffered in action in Basra.
Iraqi Vice President Tariq al-Hashimi, a Sunni Arab, has made a joint televised appeal for calm with President Jalal Talabani, a Kurd, and Shi'ite leader Abdul Aziz al-Hakim.