Foreign ministry spokesman Abbas Mousavi, quoted by state news agency IRNA, condemned the attack and "demanded decisive, effective and responsible action... against destructive agents and aggressors".

"Iran has officially communicated its disgust to the Iraq ambassador in Tehran," he said.

The Najaf consulate was set alight late Wednesday during anti-government protests, sending tall flames and thick clouds of smoke into the sky, an AFP correspondent reported.

The protesters broke into the building itself, which had been apparently evacuated of its Iranian staff.

"Victory to Iraq!" and "Iran out!" they chanted against the country they accuse of propping up a government they have been demonstrating against for nearly two months.

Iran's consulate in Iraq's second holy city of Karbala was targeted earlier this month, and security forces defending the site shot four demonstrators dead at the time.

Iran and Iraq have close but complicated ties.

The two countries fought a devastating 1980-1988 war, but Iran now has significant sway among Iraqi political and military leaders.