The transfer of security control from a US-led international mission to Afghan troops has reached an irreversible phase, a top NATO commander said Tuesday, as he outlined moves to stem insider attacks on NATO forces.

"We have now reached a phase where the transition is irreversible," said Germany's General Wolf Langheld, chief of NATO's Allied Joint Force Command (JFC), based in Brunssum in the southeastern Netherlands.

"Afghan police and security forces are now in the lead to provide security in 75 percent of the country," he told journalists.

Afghan President Hamid Karzai in May announced a new transfer of security control from NATO that would see local forces take responsibility of three-quarters of Afghanistan's population.

It is the third phase of the transition of military control in the war-torn country and another step towards the eventual withdrawal of 130,000 US-led NATO troops by the end of 2014.

ISAF expected its forces in Afghanistan are to decreased to just under 100,000 by the end of the year, the JFC's deputy chief-of-staff, Major-General Joseph Reynes Jr. said, but how many would remain after 2014 still had to be decided.

Langheld, who commands the International Security Assistance Force (ISAF) in Afghanistan's operational headquarters, said so-called "green-on-blue" attacks were of "grave concern".

"We have introduced a whole range of measures," he said, which included new training for young Afghan soldiers and their NATO counterparts to understand cultural differences.

An Afghan army soldier on Monday killed two NATO troops in the latest "green-on-blue" attack, taking the death toll from insider attacks to 12 alone this month and a total of 42 this year. This accounted for around 13 percent of all NATO deaths in 2012.

NATO has struggled to stem such attacks and they have become a major issue in the Afghan war, eroding trust between the two forces.

The growing number of attacks is likely to add to pressure in NATO nations for an early exit from the increasingly unpopular conflict, now nearly 11 years old and America's longest war.

NATO has been training a 350,000-strong Afghan security force to take over and is quick to praise their growing skills in coping with the Taliban, but officials acknowledge that challenges remain in the transition process.

Among them are Afghan government corruption, a weak state and the lack of a properly functioning justice system -- giving rise to widespread fears of a new civil war when the Western forces leave.

"There will be tough fighting to be done right until the end of 2014," said Langheld.