NATO has ordered a cutback  on operations alongside Afghan forces in response to a surge of "insider attacks" on foreign servicemen, a decision that could complicate plans to hand security over to Afghan forces ahead of a 2014 drawdown.

The order, issued by the second most senior U.S. commander in Afghanistan, Lieutenant-General James Terry, indefinitely suspends joint operations for units smaller than 800-strong battalions, where most training and mentoring takes place.

"The need for that will be evaluated on a case by case basis and approved by regional commanders," said Major Adam Wojack, a spokesman for the 100,000-strong NATO-led force backing the Afghan government against Taliban insurgents.

The order, Wojack said, would impact on the "vast majority" of the 350,000 members of the Afghan security forces who will now have to operate without support from NATO-force allies. That will deal a blow to NATO's longstanding focus on training.

At least 51 members of NATO's Afghan force have been killed in insider attacks this year, in which Afghan police or soldiers have turned their weapons on their Western mentors. That represents a spike of more than 40 percent on similar incidents for the whole of last year.

The order was issued after weekend attacks by Afghan police in which six foreign soldiers were killed in the south, where the Taliban draw most support.

Wojack said Afghan forces had already taken responsibility for security operations in many areas, including districts with a strong insurgent presence, while operations could be approved on a case-by-case basis.

"This does not mean there will be no partnering below that level," he said.

The attacks have already prompted several coalition members, including France, to speed up or review plans to withdraw troops ahead of the 2014 timetable for most combat forces, as agreed by the government's Western backers.

U.S. Defense Secretary Leon Panetta told a new conference in Beijing on Tuesday the U.S. government was concerned about the insider attacks but the plan to hand over security to Afghan control by 2014 remained in place.

Afghan commanders were not told of the order until Tuesday, in a hurried meeting with NATO counterparts. That underscored a scramble among coalition countries to contain the damage caused by insider attacks both on front-line troop morale and on fading support at home for the 11-year war.

"We haven't heard officially from foreign forces about it," Afzal Aman, head of operations for the Afghan defence department, told Reuters.

The Pentagon said in a statement the decision was reached with key Afghan leaders.

"NEGATIVE IMPACT"

The order to curtail joint operations would hobble support from NATO for Afghan military operations at a time when the Taliban were stepping up attacks, Aman said, including a raid on a major foreign force base in Helmand on Friday which destroyed more than $200 million worth of Harrier fighter jets.

"It will have a negative impact on our operations. Right now, foreign forces help us in air support, carrying our personnel, wounded and dead out of the battlefields, in logistics and training," he said.

The order still allows major joint operations above battalion size to take place, but these are less frequently conducted than smaller platoon and squad-size missions.

It could also complicate tense negotiations between Washington and Kabul on a deal to keep some special forces and trainers in the country after 2014, a sensitive topic for Afghans embittered by continued civilian deaths and more than a decade of war.

Officials from both sides had hoped to conclude a deal by early next year, but Afghan President Hamid Karzai this week accused Washington of breaching previous security agreements underpinning the talks.

Karzai's office also denounced a NATO weekend air strike in which at least eight women collecting firewood were killed east of the capital.

"This is not a happy day for the coalition," said an ISAF official, speaking on condition of anonymity. (Additional reporting by Mirwais Harooni and David Alexander; Editing by Rob Taylor and Ron Pop