US General Joseph Dunford made the remarks during a meeting on Friday in Kabul with a group of foreign correspondents based in the Afghan capital.
He asked the international community not to abandon Afghanistan after foreign forces occupying the country leave next year.
"The gains that we have made to date are not going to be sustainable without continued international commitment," Dunford stated. "We are not where we need to be yet".
"The continued presence of the international community politically, in development and in security is necessary to sustain the progress that we have made.
"Is the effort we have had to date sustainable? I would say no, not until we develop the procedures (and) the institutions necessary to sustain a modern national army."
According to the website icasualties.org, 3,335 foreign soldiers have been killed in Afghanistan since the US-led war began in October 2001.
Thousands of Afghan civilians, including a large number women and children, have been killed during night raids by foreign forces and CIA-run assassination drone strikes.
The increasing number of casualties in Afghanistan has caused widespread anger against the US and other NATO member states, undermining public support for the Afghan war.
The US-led war in Afghanistan removed the Taliban from power, but insecurity continues to rise across Afghanistan, despite the presence of about 100,000 US-led troops.