The last of the 33,000 surge troops ordered into Afghanistan by President Barack Obama more than three years ago have withdrawn from the country, returning the US presence to pre-surge levels.

The US defence secretary, Leon Panetta, said the surge had accomplished its objectives of reversing the Taliban momentum on the battlefield and dramatically increase the size and capability of the Afghan national security forces.

The withdrawal, which leaves 68,000 American forces in the warzone, comes as the security transition to Afghan forces is in trouble, threatened by a spike in so-called “insider attacks” in which Afghan army and police troops, or insurgents dressed in their uniforms, have been attacking and killing US and Nato forces, Associated Press reported.

The attacks have called into question the core strategy that relies on Nato troops working shoulder to shoulder with Afghans, training them to take over the security of their own country so the US and its allies can leave at the end of 2014 as planned.

Military commanders say the war strategy is on track and that they have made broad gains against the Taliban, wresting control of areas where the insurgents once had strong footholds. Panetta has characterised the insider attacks as the last gasp of a desperate insurgency.

But other top military leaders, including US general Martin Dempsey, chairman of the joint chiefs of staff, are worried about the impact of the attacks on the troops. Dempsey called them a “very serious threat” to the war campaign and has declared that “something has to change”.

Besides, while the Taliban often claim they have infiltrated the Afghan security forces and are carrying out these attacks, the U.S. Defense Department has recently published a report that maintains the attacks are not carried out by insurgents.

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