Sadr's gains have called into question the presence of US forces in Iraq, where more than 5,000 troops are indefinitely deployed to prevent a resurgence of the Islamic State group.

"The Iraqi people had an election. It's a democratic process at a time when people, many people doubted that Iraq could take charge of themselves," Mattis told Pentagon reporters.

"So we will wait and see the results -- the final results of the election. And we stand with the Iraqi people's decisions."

After the 2003 US-led invasion, Sadr's Mahdi Army militia battled US forces, and he is now calling for American troops to leave following last year's defeat of IS.

Mattis, a retired four-star Marine general, served in Iraq and went on to lead the US Central Command that runs military operations in the Middle East and Afghanistan.

As a Marine commander, he saw US troops killed by the Mahdi Army, mainly in the poor Baghdad district of Sadr City and in the holy city of Najaf, further south.

It is not yet clear who will be Iraq's next prime minister, with Sadr eyeing a governing coalition.

Washington's preferred candidate, Prime Minister Haider al-Abadi, has been knocked into third place. Sadr has ruled himself out of becoming prime minister.

While his family of religious scholars historically has close ties with the Islamic revolutionaries in Iran and he spent years living there, Sadr has now fallen out with Tehran and wants its overbearing influence slashed.

Sadr and his militia played central roles in the wave of sectarian bloodshed that peaked in 2006-2007, but he eventually froze the militia's activities in a move the US credited with sharply reducing violence.

The Pentagon said it was too soon to know what the results of the election might mean for the US troop presence.

"We do not support any particular candidate or party; rather we support a fair and transparent process," Pentagon spokesman Eric Pahon said.

"We stand ready to work with whoever is fairly elected by the Iraqi people."