US President Donald Trump has questioned the decision of his predecessors to first send the American troops to Iraq and then withdraw them, while assuring support to the war torn country to maintain stability, the Press Trust of India (PTI) reported.

"I know Mosul is moving along, but Mosul was ours until we left," Trump told reporters during a brief media appearance with the visiting Iraqi Prime Minister Haider al-Abidi in the Cabinet room of the White House.

"So perhaps we shouldn't have gone in, and certainly we shouldn't have left. We should never ever have left, and the vacuum was created, and we discussed what happened," Trump said after the two leaders met in the Oval office.

Trump said his main thrust is to get rid of ISIS (Daesh).

"We're going to get rid of ISIS. It will happen. It's happening right now," he said.

The US Defence Secretary General (retd) James Mattis and his team have done an incredible job, he said.

"A lot of things are different than they were even five or six weeks ago. We've been here a short while and people have said they'd never seen such a difference," Trump said.

In his remarks, al-Abidi said ISIS is a terrorist organisation which threatens the security of the world.

Speaking in Arabic, the Iraqi Prime Minister emphasised the fact that Iraq is on the forefront of fighting ISIS.

The Iraqi Prime Minister said that he wants to build on the Strategic agreement Iraq signed with the US.

The Prime Minister and his delegation are spending a few days in Washington, during which he is expected to meet with congressional leaders on the hill.

Meanwhile US Senators Bob Corker and Ben Cardin, the chairman and ranking member of the Senate Foreign Relations Committee, led a bipartisan letter to Trump urging continued US support to foster unity and stability in Iraq.

The letter was supported by more than a dozen other Senators.

"We urge you to continue working with Iraq's leaders to preserve their country's unity and ensure its stability.

This requires a plan for decentralising some functions of the government, disbanding Iranian-aligned militias, addressing the humanitarian needs of Iraq's community, and committing to a program of reconciliation," the letter said.

Later, in an address to a Washington think-tank community, al Abidi said that Trump has assured Iraq's government of greater US support as it confronts ISIS and stabilises the country.

"We have been given assurances that the support will not only continue but will accelerate," he said in his address to the US Institute of Peace, a top American think-tank.

However, the US support is unlikely to include the United States "sending troops in large numbers" to Iraq.

"I think this administration wants to be head to head with terrorism, to be more engaged. We are not talking about (US) military confrontation as such in Iraq," he said.

According to al-Abidi, Iraq's forces "are at the last stage of liberating" the city of Mosul and its province from fighters of the Islamic State, or ISIS.

Later this week, the Iraqi Prime Minister will also attend the March 22-23 conference hosted by the State Department to frame strategy for the 68-member Global Coalition to Counter ISIS.

"We need to have a plan together. We shouldn't be derailing the whole thing" or losing focus against ISIS because of clashes among the region's major actors," he said.

-- BERNAMA