His remarks are the latest example of policy differences with US president

The US military is not in Iraq "to seize anybody's oil", Defense Secretary Jim Mattis said, distancing himself from remarks by President Donald Trump at the start of a visit to Iraq on Monday.

Mattis, on his first trip to Iraq as Pentagon chief, is hoping to assess the war effort as US-backed Iraqi forces launch a new push to evict Islamic State militants from their remaining stronghold in the city of Mosul.

But he is likely to face questions about Trump's remarks and actions, including a temporary ban on travel to the United States and for saying the country should have seized Iraq's oil after toppling Saddam Hussein in 2003.

Trump told CIA staff in January: "We should have kept the oil. But OK. Maybe you'll have another chance."

Mattis, however, flatly ruled out any such intent. "We're not in Iraq to seize anybody's oil," he told reporters traveling with him.

His remarks are the latest example of his policy differences with Trump. The president has acknowledged that Mattis did not agree with him about the usefulness of torture as an interrogation tactic but, in a sign of Mattis' influence, said he would defer the matter to his defense secretary.

Mattis has also been more critical than Trump of Russian President Vladimir Putin, and distanced himself from Trump's labeling of the media as "the enemy of the American people", saying he had no problems with the press.

A retired Marine general who led US troops in Iraq, Mattis has also sought an exemption from Trump's travel ban for Iraqis who served with US troops, including translators.

He said he had not seen a new executive order which the administration is considering. "But I right now am assured that we will take steps to allow those who have fought alongside us, for example, to be allowed into the United States," Mattis said.

Mosul campaign

Mattis is finalizing plans at Trump's request to accelerate the defeat of Islamic State and is expected to meet senior US and Iraqi officials in Iraq.

His visit came a day after Iraqi Prime Minister Haider al-Abadi announced the start of the ground offensive on western Mosul, where Islamic State militants are essentially under siege along with an estimated 750,000 civilians.

The insurgents were forced out of the east of the city last month after 100 days of fighting.

The US commander in Iraq, Army Lieutenant General Stephen Townsend, has said he believes US-backed forces will recapture both of Islamic State's major strongholds - Mosul and the city of Raqqa in Syria - within the next six months.

The defense secretary's strategy review could lead to additional deployment of US forces, beyond the less than 6,000 US troops deployed to both Iraq and Syria.