The US has announced the end of its 23-year military presence in Iraq, as it shifts its focus to expanding the war against Iran.

US President Donald Trump and Iraqi Prime Minister Ali al-Zaidi said Tuesday at the White House that US forces will leave Iraq by September 30, ending a deployment that began with the 2003 invasion and the overthrow of then-Iraqi President Saddam Hussein, and later continued under the banner of counterterrorism.

It is believed that around 2,000 American soldiers are still stationed in Iraq.

"We don't think we need the military there anymore," Trump said, arguing that the security environment in Iraq has changed after months of US-Israeli attacks "destabilized" Iran.

Al-Zaidi confirmed that US forces will leave by the end of September and that "American companies" will enter in their place.

Trump said the relationship with Iraq would now focus on investment and energy, citing the country's oil reserves. He said the two countries "will do a lot of deals" and that the US "will be producing a lot of oil."

The US invaded Iraq in March 2003, claiming that Saddam Hussein possessed weapons of mass destruction and had links to terrorist groups. Although the weapons were never found, the US presence grew to more than 170,000 troops by 2007.

In 2011, most combat forces left, but the US returned in 2014 after the Islamic State (IS, formerly ISIS) overran large areas of Iraq and Syria.

Critics argued that the invasion of Iraq was launched on false pretexts, destabilized the country, helped create the conditions for the rise of ISIS, and was “mostly about oil,” which former Federal Reserve Chairman Alan Greenspan acknowledged in his memoirs.

Trump has been much more outspoken about this than past administrations, arguing that America should have "taken the oil" in Iraq and that US forces in Syria would "hold the oil."