In testimony before the Senate Armed Services Committee, Army Gen. John Campbell said, "Now, more than ever, the United States should not waver in Afghanistan."
President Barack Obama has previously announced that the U.S. will reduce its military footprint from 9,800 troops to 5,500 by the start of next year.
But Campbell warned the committee that "Afghanistan has not achieved an enduring level of security and stability that justifies a reduction of our support in 2016."
Campbell said that building up the Afghan military was "akin to building an airplane while in flight" and said a reduction to 5,500 troops would only allow for a "very limited" training mission.
Currently, the U.S. troops stationed in Afghanistan are split between the NATO-led training and assist mission, Operation Resolute Support, which accounts for 6,800 American troops, and a separate mission tasked with performing counterterrorism operations.
Responsibility for combat operations was transferred to the Afghan National Security Forces in 2014.
It has been a difficult year for the Afghan army, which has suffered heavy casualties in its battle with the Taliban insurgency. The Taliban control or influence as much as 20% of Afghanistan, its highest levels of control since 2001.
Experts, including the former commander of U.S. forces in Afghanistan, retired Gen. David Petraeus, have called for increased U.S. airstrikes to bolster the Afghans in their fight against the Taliban. The U.S. has been prevented from providing close air support to the Afghan military due to rules of engagement that were enacted following the end of U.S. combat operations.
The rules only permit coalition airstrikes in situations where coalition troops are directly threatened and as part of counterterrorism missions against transnational groups like al Qaeda. Last month, U.S. forces were granted the authority to strike ISIS
in Afghanistan. Campbell said that this authority was granted one year after the coalition became aware of ISIS's presence in Afghanistan.
Campbell acknowledged a steep drop in close air support, noting that in prior years, the U.S. had provided 150 attack helicopters and two Air Force squadrons.
Campbell described the indigenous Afghan air capability as being "extremely limited," with its airstrike capability having dropped to three Indian-supplied attack helicopters.
"Close air support has been the one resource and capability that the Afghans have asked me for every single day," Campbell said.
Campbell said he plans to emphasize the need to fill this air support gap in his final recommendations to the Obama administration, which could include a loosening of restrictions on U.S. airstrikes.
"Those who serve in Afghanistan understand it's worth the investment," Campbell said.