Addressing reports that Trump plans to announce within days sharp cuts to US forces in both countries, the Republican Senate chief said the US would be "abandoning" partners.

He added that the move would leave room for the Taliban to take control of Afghanistan and the Islamic State group and Al-Qaeda to rebuild.

"The consequences of a premature US exit would likely be even worse than president Obama's withdrawal from Iraq back in 2011, which fueled the rise of ISIS and a new round of global terrorism," McConnell said on the Senate floor.

"It would be reminiscent of the humiliating America departure from Saigon in 1975."

McConnell spoke after media reports said the Pentagon had received directives to prepare to bring back another 2,000 US forces from Afghanistan and 500 from Iraq before Trump steps down on January 20.

That would leave only about 2,500 troops in each country, fewer than US military officials have said is enough to ensure stability.

Before he was fired on November 9, defense secretary Mark Esper had insisted on keeping 4,500 troops in Afghanistan until the Taliban reduced its attacks on the Afghan government to show a commitment to peace talks.

The Pentagon did not confirm the reports, which said Trump could announce the withdrawals this week.

Trump entered office nearly four years ago pledging to end US wars abroad.

He has appeared determined to get close to that before he steps down, following his November 3 election loss to Democrat Joe Biden.

He said last month he wanted US troops home "by Christmas" and his National Security Advisor Robert O'Brien refined that to mean the Afghan presence would fall to 2,500 as early as January.

McConnell, normally a close political ally of Trump, warned a rapid withdrawal would "delight the people who would wish us harm."

"The spectacle of US troops abandoning facilities and equipment, leaving the field in Afghanistan to the Taliban and ISIS, would be broadcast around the world as a symbol of US defeat and humiliation, and a victory for Islamic extremism," he said.

"It would hand a weakened and scattered Al-Qaeda a big, big propaganda victory and a renewed safe haven for plotting attacks against America."