The US-trained Afghan army is supposed to take over the security responsibilities now carried out by foreign troops.
Army Major-General Robert Durbin, who heads the US effort to train and equip Afghan government security forces, said the national army numbers "a little bit over 30,000" and that it is growing at a rate of 1,000 a month.
The plan is to have 70,000 Afghan soldiers trained in three years.
It has been almost five years since the Taliban were defeated in Afghanistan and US forces have been helping build a new national army from scratch.
Durbin, the commander of Combined Security Transition Command-Afghanistan, told a Pentagon briefing that Afghan security forces were making steady progress, but problems with absenteeism and developing capable leaders remain "a challenge".
He said 3,500 US troops out of an American force of 23,000 in Afghanistan are dedicated to training the Afghan army and police.
The general said three years was needed to recruit enough quality personnel.
Some of the first Afghans who volunteered for three-year stints in the army are reaching the end of their service, and about 35% are re-enlisting, well below the goal of 50%.
Durbin said there are about 62,000 police officers in Afghanistan. About 58,000 are considered trained but only 37,000 are considered equipped. He said 86,000 vehicles are needed for the national police, and there are only 2,000 now.
It will take "at least the next year or two" to get the police force fully trained and equipped.
Durbin added that US trainers are working to remove corruption from the police force.
"There are perhaps many bad lessons or behaviours that these policemen have learned, and they don't understand the true essence of rule of law and to serve and protect."
Abdul Rahim Wardak, the Afghan defence minister, earlier this week said an Afghan army of 150,000 to 200,000 would be needed to secure the country.
Durbin said that while the plan was for an army of 70,000, he did not rule out a larger force sometime in the future.