Crowds thronged the streets of Najaf on Wednesday as the coffin of Iran's supreme leader Ali Khamenei moved through the holy city in a procession devoted to Iraq, home to Shia Islam's most sacred shrines.

Iran began six days of public funeral ceremonies for Khamenei on Saturday, including a dedicated day to neighbouring Iraq, a Shia powerhouse with close ties to Tehran.

The Islamic republic hopes the marathon ceremonies will project strength and unity after the Middle East war, which started with US-Israeli strikes that killed Khamenei and several relatives on February 28.

The procession in Najaf came as the United States and Iran renewed hostilities in the Strait of Hormuz, putting more pressure on a deal to end the war.

The US military said it had struck dozens of Iranian targets in response to Tehran's attacks on three ships in Hormuz, with Iran's Revolutionary Guards later saying they had hit US military facilities in Bahrain and Kuwait.