US Defence Secretary James Mattis has said that the withdrawal of US troops from Afghanistan was done too rapidly.
Mr Mattis was briefing Nato allies on future strategy, amid reports the US will boost its military presence.
His comments were in direct contrast to Nato's secretary-general, who said the withdrawal should have happened sooner.
US troops reached 130,000 in 2011 but were drawn down, leaving the Afghan military in control at the end of 2014. There are now 13,500 Nato troops there.
Mr Mattis was speaking at a press conference after meeting Nato defence ministers in Brussels.