The secretary-general "has been following with concern the ongoing protests in Iraq today, during which demonstrators entered government buildings," Stephane Dujarric said in a statement.

"He appeals for calm and restraint, and urges all relevant actors to take immediate steps to de-escalate the situation and avoid any violence," Dujarric added.

"The Secretary-General strongly urges all parties and actors to rise above their differences and to engage, without further delay, in a peaceful and inclusive dialogue on a constructive way forward."

Baghdad's Green Zone was rocked by violence Monday after powerful Shiite cleric Moqtada Sadr said he was quitting politics, sparking chaos in which 12 of his supporters were killed.

Tensions have soared in Iraq amid a political crisis that has left the country without a new government, prime minister or president for months.

The situation escalated sharply after Sadr's supporters stormed the government palace on Monday following their leader's announcement.

By evening at least seven shells had fallen in the high-security Green Zone, which houses government buildings and diplomatic missions, a security source told AFP on condition of anonymity.

It was not immediately clear who was behind the shelling, which was followed by the sound of automatic weapons being fired in the Green Zone.