US President Donald Trump wants combat forces reduced in Afghanistan by the next US presidential election, Secretary of State Mike Pompeo said on Monday, in comments that underlined the growing pressure from Washington to cut troop numbers there.

Trump's South Asia strategy, unveiled in August 2017, called for an open-ended deployment of US forces with the goal of compelling the Taliban to negotiate peace with the Kabul government to end nearly 18 years of war.

However, Pompeo's comments underscored a shift that has apparently taken place since talks with the Taliban opened last year.

"That's my directive from the president of the United States," Pompeo told The Economic Club of Washington DC when asked whether he expects the US to reduce troops in Afghanistan before the next election in November 2020.

"He's been unambiguous: end the endless wars, draw down, reduce. It won't just be us," he said, referring to Trump's directive. "We hope that overall the need for combat forces in the region is reduced."

The disclosure of a timeline will add to speculation that Trump is prepared to strike any deal with the Taliban that will allow for at least partial US withdrawal before American voters go to the polls, irrespective of the concerns of the US-backed government in Kabul.

"Now suddenly adhering to the date suited to Trump's election date has become more important than the meticulous task of bringing peace to Afghanistan," said a senior Afghan official who is also a close aide to President Ashraf Ghani.

"The American haste to pull out foreign troops has only provided more leverage to the Taliban. Afghan forces will be soon abandoned to fight the war alone," said the official, who declined to be identified.

Pompeo's comments come at a delicate moment, as Afghanistan prepares for a presidential election of its own in September and the United States prepares to engage in another round of talks with the insurgents.

More than 20,000 US and other NATO coalition troops are in Afghanistan as part of a mission to train, assist and advise Afghan forces.