Afghanistan's fledging security forces will not be sustainable as they take on the Taliban unless major reforms are pushed through the government in Kabul, a senior NATO commander warned Tuesday.

Lieutenant General John Lorimer, deputy commander of the NATO coalition in Afghanistan, said the national army and police were now at nearly full strength, but their long-term effectiveness remained uncertain.

"The Afghan security forces' full complement of 352,000 has almost been achieved. We now need to concentrate on quality because what we have today is not yet fully sustainable," the British general told reporters.

"We need to assist the Afghan ministry of defence and ministry of interior in adapting, restructuring and developing capacity.

"Currently neither of those institutions has the necessary end-to-end processes to generate enduring capability."

Lorimer said that NATO coalition leaders had to ensure that the Afghan government learned the necessary budgeting, planning and management skills to keep the country's security forces active on the battlefield.

"We are now very much focusing on this key area," said Lorimer, who took up his post last month.

All 87,000 NATO-led combat troops in Afghanistan will leave by the end of 2014, and the Afghan army and police have taken the lead in all operations to thwart the insurgents who have fought a fierce guerrilla war for 12 years.

NATO troop numbers currently drop by more than 2,000 a week, and many Afghans fear a return of nationwide violence and turmoil as the Taliban ramp up efforts to regain power after they were ousted in 2001.

"Can the ANSF (Afghan National Security Forces) effect a full security transition in December 2014? The answer would be a qualified yes," said Lorimer, as Afghan troops battle the insurgents during the often violent summer months.

"They will emerge out of the fighting season confident and capable, and we will try to get them being sustainable as well into the future," he added.

"There is absolutely no comparison at all between the capability of the ANSF in 2007 and what they are now."

Pressure is increasing on Afghan and international leaders to negotiate a peace deal with the Taliban to foster stability before the NATO troops depart, but efforts to start negotiations in Doha, Qatar failed badly in June.

The Taliban opened an office in the Gulf state that enraged Afghan President Hamid Karzai by styling itself as an unofficial embassy for a government-in-exile, triggering a major diplomatic bust-up.

Karzai broke off bilateral security talks with the Americans and threatened to boycott any peace process altogether.

US Lieutenant General Mark Milley described the Taliban as an "aggressive and ruthless" enemy and said that political efforts to bring the insurgents to the peace table had no impact on the military campaign to crush them.

"This thing that occurred in Doha, operationally it has no impact on us," he said. "We are not getting instructions either way, we are not doing negotiations, we are the military, we are out there conducting tactical operations."

Taliban leader Mullah Omar last week dismissed presidential elections due in April as "a waste of time" but insisted the militants had no desire to grab power after NATO troops leave.

The US is considering leaving a residual military force in Afghanistan to aid stability, target Al-Qaeda fighters and further train up the national security forces.