NATO military commanders will beef up security measures to prevent insurgents from infiltrating the Afghan army after French troops were killed by a renegade soldier, the alliance chief said Friday.

NATO defence ministers endorsed a French proposal to task military authorities with devising new plans before the end of the month as they wrapped up two days of talks focused on the decade-old war in Afghanistan.

"We have already taken a lot of steps," NATO Secretary General Anders Fogh Rasmussen told a news conference. "But in light of recent events we agreed to strengthen those efforts."

Six percent of overall NATO deaths in Afghanistan have been attributed to attacks by Afghan security forces, according to a confidential alliance report leaked to the media last month.

Some 130,000 NATO troops work with more than 300,000 members of the Afghan security forces.

"There is a realisation about the need to better control the recruitment to prevent infiltrations in the army," French Defence Minister Gerard Longuet told reporters.

"These events are rare but they are symbolically important for the credibility of the Afghan army," he said. "Quantitatively, they are marginal. Media-wise, they are unbearable."

Some 40 attacks were committed by Afghan forces against NATO troops in the last four years, including 18 last year, Longuet said.

NATO did not provide details about the plans but they are expected to focus on better controlling the recruitment of Afghans, deeper investigations into potential recruits and the use of biometric technology.

The killing of four French unarmed soldiers at the hands of an Afghan they were training last month prompted French President Nicolas Sarkozy to end his country's combat role in Afghanistan by the end of 2013.

France will gradually withdraw its 3,600 troops, eventually leaving behind around 400-500 military trainers at the end of 2014, when NATO is scheduled to end its combat mission, Longuet said.

NATO defence ministers voiced hope on Thursday that Afghan forces will take the lead by the end of next year, while foreign troops move to a backup role until their combat mission ends at the end of 2014.

The ministers turned their attention to the future size of the Afghan army, and cost of maintaining it, on the second day of talks Friday.

The goal is to increase the size of the security forces to about 350,000 by October. But some allies say the future size of the force could be smaller, with Longuet suggesting 230,000 as a good number.

The United States has forecast that the annual price tag of training and equipping Afghan security forces in coming years to be around $6 billion (4.6 billion euros).

NATO leaders are due to decide on the future size and cost of maintaining Afghan forces at a summit in Chicago in May.