Taliban insurgents will increase the number of insider attacks against coalition and Afghan forces, which have resulted in the deaths of at least 52 foreign troops so far this year, the movement's reclusive leader said Wednesday.

In an emailed statement congratulating Muslims as they prepare to celebrate the Eid al-Adha holiday, Mullah Mohammad Omar urged "every brave Afghan in the ranks of the foreign forces and their Afghan hirelings ... to strike them."

"Jihadist activities inside the circle of the state militias are the most effective stratagem. Its dimension will see further expansion, organization and efficiency," he said. "Increase your efforts to expand the area of infiltration in the ranks of the enemy."

Also Wednesday, NATO said in a statement that two of its soldiers died following an insurgent attack in southern Afghanistan. It did not provide further details about the attack or the nationality of the victims.

The surge in insider attacks is throwing doubt on the capability of the Afghan security forces to take over from international troops ahead of a planned handover to the Afghans in 2014. It has further undermined public support for the 11-year war in NATO countries.

The attacks have not been limited to members of the NATO-led international coalition. More than 50 Afghan members of the government's security forces also have died this in attacks by their own colleagues.

The Taliban leader also claimed his fighters were winning the war and vowed to continue the struggle "against the invaders who have invaded our country until the occupation ends completely."

"We told the enemy 11 years ago that their coming (to the country) will be easy but their presence and exit will be full of complications," he said.

Omar also exhorted his fighters to "pay full attention to the prevention of civilian casualties," saying the enemy was trying to blame them on the insurgents.

Last week the United Nations called on the Taliban leadership to stop the use of homemade roadside bombs and mines. The Taliban say they use only remote-controlled roadside bombs which — unlike the mines automatically activated by pressure-plates — allow a bomber to choose the time of the blast and specifically target coalition troops and their Afghan allies.

But the world body says that insurgent-placed homemade bombs, also known as improvised explosive devices or IEDs, continue to be the deadliest weapon for civilians. IEDs killed 340 civilians and injured a further 599 over the past nine months, an increase of almost 30 per cent compared to the same period last year, the UN said.