The commander of NATO's mission to train Afghanistan's security forces has warned it will need years of support from foreign powers and the Afghan government to be a long-term success.

Lieutenant-General William Caldwell indicated that several thousand international trainers could be needed to support the mission until at least 2020 in an interview with AFP.

All foreign combat troops will leave Afghanistan by the end of 2014 in a process that started last month and will require the Afghan army and police to play an ever-greater role in fighting the Taliban insurgency.

But Caldwell also said that President Hamid Karzai's government needed to make progress on governance and justice in Afghanistan to help sustain the efforts of NTM-A (NATO Training Mission Afghanistan).

"I'm very confident that the Afghans can in fact take the lead for security by December 2014 -- there's no question they can do it," Caldwell said.

"But it will require two things in order to make sure that happens. One is the international community has to remain committed to this mission.

"If we start a precipitous withdrawal of monetary and trainer support, that could impede that ability.

"And the second thing is the Afghan government has to be committed to this. Right now they are, but if for some reason they waver and they don't continue to do what they're doing then that would call into question whether we can make it happen."

The US three-star general is expected to leave Afghanistan within a couple of months, after leading NTM-A since its establishment in late 2009.

His job is seen as one of the most important in Afghanistan and NTM-A has a budget of $11.6 billion for this year alone.

The Afghan army and police have been recruited and trained rapidly, growing from around 190,000 in late 2009 to 305,000 today. That figure is due to rise again to 352,000 by November 2012.

Despite the US emphasis on quantity over quality in the Afghan forces earlier in the war, which is now in its 10th year, Caldwell insisted progress had been made on improving training.

But the Afghan security forces have faced persistent questions over issues such as cronyism and retention levels.

Meeting new recruits during a visit to training facilities in the city of Herat, western Afghanistan, Caldwell was visibly proud of how the training mission has developed under his stewardship.

Caldwell acknowledged afterwards, though, that their ultimate success was also reliant on external factors.

The security forces "require international support for many more years" in helping to develop as properly functioning institutions, rather than on the fighting side, he said.

He put the figure for this at "maybe 3,000 people, uniform-type people, police and army" plus financial support to help the Afghan government pay for the security forces, possibly for another six years.

Some diplomats and Western officials in Kabul suggest it could be up to 10 years before the Afghan government can afford to fund its own security forces.

"It does take about a decade to fully implement things wherever you are in the world," Caldwell said.

"We're just now starting to put it into place here in 2011. So if you use that as a model, that means you would probably be out to 2020 with some presence here."

He also stressed that the Afghan government had to build up good governance and justice systems to help improve security.

Caldwell said the Afghan forces still had some way to go, notably in improving literacy rates, which would allow troops and police to take notes in class, read instruction manuals and keep records of incidents.

Only 14 percent of new recruits are functionally literate, but NTM-A says it is on track to get 50 percent of the security forces reading and writing at a level equivalent to that of an American eight-year-old by the end of this year.

Caldwell was optimistic that the Afghan security forces can overcome the challenges facing them to help create a brighter future for their long-troubled country.

"I see hope here that I probably didn't see two years ago," he said. "They can do this mission. They'll need resources but they can do this mission."