Mr Pompeo announced the cut following a trip to the Afghan capital, Kabul, where he failed to break a deadlock between two politicians who both claim victory in the presidential election.
He hoped to save a deal signed between the Taliban militant group and the US.
The agreement is supposed to pave the way to peace in Afghanistan.
However, the political disunity in Kabul has hampered efforts to create a negotiating team and the talks with the Taliban – a key part of the agreement signed late last month – have yet to begin.
Since the US intervened in 2001 to dislodge the Taliban, tens of thousands of people have been killed in Afghanistan, including an estimated 32,000 civilians.
The US and its Nato allies have now agreed to withdraw all troops within 14 months – if the Taliban abide by the deal.
What is blocking the peace process?
A political row between Ashraf Ghani – who has served as president since 2014 – and his rival, Abdullah Abdullah, who served as the country’s chief executive in the last administration.
Both stood as candidates in the presidential election last September, with Mr Ghani officially declared the winner in February 2020 – a result challenged by Mr Abdullah.
Both men have since claimed victory and held rival inaugurations earlier this month.
Mr Pompeo visited Kabul in an…