Amid increasing anger at frequent attacks on NATO troops by Afghan security forces, former Danish prime minister Anders Fogh Rasmussen has been granted a fifth year in office as the secretary general of NATO. His term will now end in July 2014.

Addressing a monthly news briefing at NATO headquarters in Brussels, Rasmussen admitted green-on-blue attacks had undermined the trust between NATO and Afghan forces and has hurt working ties between them.

"I am mad as hell," said General John Allen, the top US commander in Afghanistan, in reaction to the sharp rise in inside attacks on the day when the US Army death toll in Afghanistan reached 2,000. Attacks from Afghan forces account for approximately 16 percent of American casualties in the country. "We are willing to sacrifice a lot for this campaign, but we are not willing to be murdered for it."

About 114 US troops have been killed in green-on-blue attacks in Afghanistan since 2007, but this year has been far worse, with 51 deaths by September. According to NATO's own analysis, a quarter of the attacks were the result of Taliban influence and infiltration.

NATO plans to withdraw most of its troops from Afghanistan by the end of 2014. Afghan security forces are expected to reach the strength of 350,000 by that time, allowing the NATO forces to end their combat role. But a large number of Afghan soldiers are instead turning their weapons against the coalition troops and trainers. NATO admits that has severely hurt their transition strategies and combat operations against the Taliban. In September, NATO decided to restrict joint operations with Afghan forces in future, indicating that its motive was to preserve the safety of its own troops.

"People ask why it is that we send trainers to Afghanistan to help the Afghan security forces, but they turn their weapons against the trainers who are there to help them," Rasmussen said during the briefing. "People, and our trainers on the ground, can't understand this."

He said he agreed with General Allen. "I have stated repeatedly that inside attacks threatened to undermine the trust and working relationship between NATO and Afghan security forces."

Green-on-blue attacks are widely seen by coalition troops as Taliban's strategy to disrupt the planned withdrawal of NATO forces by making it too dangerous for them to train and fight alongside Afghan security forces. Rasmussen admitted such attacks had also turned public opinion against the Afghanistan mission at a crucial time.

Training Afghan forces to take the lead in security responsibilities is the most vital part of NATO's transition strategy. But critics are questioning whether Afghan forces would be able to cope with the post-withdrawal threats and challenges. If Taliban's growing influence in Afghan security forces and flaws in troop vetting procedures have started to reveal whilst NATO is still in control, critics say, how will the international coalition partners ensure it would not be a cakewalk into Kabul for the Taliban after NATO's withdrawal?

Rasmussen says NATO has strengthened the recruitment and vetting procedures for Afghan security forces and these procedures are reviewed regularly. Hundreds of Afghan troops have been sent home in recent re-vettings, but background checks on all Afghan troops may not be possible by the planned withdrawal date. Vetting also involves recommendations from local tribal elders - a process that has serious limitations.

In its latest report on Afghanistan, the International Crisis Group warned Afghan security forces were not prepared for security responsibilities in the coming years, and NATO's departure in 2014 could be followed by a government collapse and a civil war.

"Steps to ensure a stable transition must begin now to prevent a precipitous slide toward state collapse," the report says. "Time is running out."

The Afghan government rejected the report as "nonsense and garbage". The NATO secretary general said he disagreed with the report and was confident that the Afghan security forces would be able to take charge after 2014.

The NATO Parliamentary Assembly will meet in November to review the progress in Afghanistan and will consider a draft report from its Defence and Security Committee that says tighter vetting procedures were introduced in February this year. NATO casualties in inside attacks have increased since then.

Adil Shahzeb is a London based international broadcaster and writer. He tweets at @adilshahzeb and can be reached at twoadilzeb@yahoo.com