"American forces on Saturday arrested Muhammad Hashim al-Yahya, coordinator of the six-person committee set up by the Marjaiya (Iraq's highest Shia authority) to supervise the drawing up of an electoral list with the backing of the ayat Allah," an al-Sistani representative in Najaf said.

"If he is not released, there will be serious consequences," the source added, giving no reason for al-Yahya's arrest in Baghdad.

The US military said it had heard reports of the arrest but was unable to give any further information.

On Tuesday, Shia political leaders said they were in the final stages of drawing up an electoral list that had al-Sistani's blessing, before the landmark elections scheduled for 30 January.