Several protesters dead in clashes in Baghdad's Green Zone

Fifteen supporters of Iraqi Shiite cleric Moqtada Sadr were shot dead in Baghdad's Green Zone on Monday in violence following his announced withdrawal from politics, medical sources told AFP in a revised toll.

Some 350 protesters have been injured, some by bullets and others by inhaling tear gas, in the chaos that has enveloped the highly secured area in the centre of Iraq's capital hosting government institutions and embassies.

Shots were fired in the fortified area which houses government buildings as well as diplomatic missions, an AFP correspondent said, as tensions soared amid an escalating political crisis that has left Iraq without a new government, prime minister or president for months.

Witnesses said earlier that Sadr loyalists and supporters of a rival Shiite bloc, the pro-Iran Coordination Framework, had exchanged fire.

The army announced a nationwide curfew from 7:00 pm (1600 GMT), and security forces later patrolled the capital.

Calling the developments "an extremely dangerous escalation", the United Nations Assistance Mission in Iraq (UNAMI) urged "all" sides to "refrain from acts that could lead to an unstoppable chain of events".

"The very survival of the state is at stake," it warned.

The United States also urged calm amid the "disturbing" reports of unrest in Baghdad.

Protests later spread to other parts of the country, with Sadr followers storming government buildings in the cities of Nasiriyah and Hillah south of Baghdad, an AFP correspondent and witnesses said, with some roads also blocked in Hillah.