Army General John F Campbell's assessment underscores the risks of Obama's longstanding goal of ending the war before he leaves office in January 2017.
The president's critics said leaving the Afghans without enough American military trainers would imperil the gains made since 2002, when the US committed to rebuilding the country. Nearly USD 64 billion has been allotted so far for building up the Afghan army and police.
Yet patience among other lawmakers is fraying with the finish line so far away. The Afghans won't be able to independently sustain their security forces until 2024, according to Campbell.
Campbell, who is expected to retire soon, is scheduled to testify Thursday before the Senate Armed Services Committee.
He appeared Tuesday before the House Armed Services Committee and described the Afghan security forces as becoming increasingly competent. Yet significant hurdles remain and "persistent" training and advising is required to overcome them, he said.
"You've got to start that now and make sure they realize if you recruit a guy now, you're not going to see him for another three years before he can be a pilot," he said.
Initially, Obama said he would reduce the US force in Afghanistan to 5,500 troops by the end of last year, and then down to 1,000 by the end of 2016. But Obama backtracked in October, saying the situation remained too fragile for such a rapid withdrawal.
During an exchange with Rep Jim Bridenstine, an Oklahoma Republican, Campbell said a force of 5,500 would be focused on conducting counterterrorism operations.
