United States President Donald Trump will announce a further withdrawal of US troops from Iraq on Wednesday, according to a senior administration official, as he faces a backlash from reports that he allegedly insulted American veterans and war dead.

He will also announce an additional reduction in US forces in Afghanistan, the official said.

Trump is trailing Democratic rival Joe Biden in opinion polls ahead of the November 3 election.

His announcement, and the timing of it, may be aimed at convincing voters that he is following through on promises to end what he has described as the US's endless wars.

The US has around 5,200 troops that were deployed in Iraq to fight the ISIL (ISIS) armed group.

A US official, speaking on condition of anonymity, told Reuters news agency in August that the country expected to reduce the number of its troops in Iraq by about a third in the coming months.

The US military returned to Iraq in 2014 to help support the government in its fight against ISIL, which had taken over large swathes of the country.

Calls for complete withdrawal

But calls for a complete withdrawal of US troops peaked and US-Iraq tensions rose following rocket attacks on US military and diplomatic sites, and the killing of Iran's elite Quds Force commander, General Qassem Soleimani, and deputy of Iraq's Hashd al-Shaabi, Abu Mahdi al-Muhandis, in a US attack near Baghdad airport in January.

Days after that incident, the Iraqi parliament voted in favour of a complete withdrawal of all foreign troops as Iran-backed armed groups vowed to avenge the killings.

What next for US troops in Iraq?

Tens of thousands of Iraqis have also rallied across the country demanding the withdrawal of US soldiers following the assassination of Soleimani and Muhandis.

The US-led coalition has since pulled out from six bases and consolidated its presence to only three bases.

In Afghanistan, the US has about 8,600 troops.

Trump said in an interview with Axios released last month that the plan was cut that number to about 4,000.