Taliban militants killed 17 Afghan soldiers in the far northeast in one of the deadliest single attacks on government security forces in recent years, officials said Wednesday.

The men were taken hostage in Badakhshan on Saturday while guarding a convoy. Their bodies were discovered by local people in the relatively peaceful mountainous province, which borders China, Pakistan and Tajikistan.

"The enemy have executed 17 soldiers whose bodies were found last night," provincial deputy police chief Sayed Jahangir Keramat said. "Seven other soldiers were released in exchange for some Taliban prisoners."

Afghan government forces have increasingly suffered the brunt of insurgent attacks as they prepare to take responsibility for national security when the bulk of NATO combat troops withdraw next year.

The attack in Badakhshan will heighten concern that Afghan forces cannot provide security across the country, where a US-led invasion ousted the Taliban regime in 2001.

"Last night locals found 17 bullet-riddled bodies of Afghan soldiers in Warduj district," Abdul Marouf Rasekh, spokesman for the provincial governor, told AFP, confirming several others had been released after talks conducted by tribal elders.

The Taliban claimed responsibility for the attack but said the deaths had occurred during fighting, rather than after the soldiers had been captured.

"The soldiers were all killed in the battlefield," Taliban spokesman Zabiullah Mujahid said in a statement sent to AFP.

"A huge amount of munitions and two vehicles were also seized," he added.

When the 100,000-strong international military coalition fighting alongside Afghan forces withdraws by the end of next year, the ability of local troops to control the insurgents is key to the country's stability.

General James Mattis, head of US Central Command, on Tuesday insisted Afghan forces were increasingly capable and that the Kabul government's army and police were now leading anti-insurgency efforts.

"There can be no longer any doubt -- it's not opinion. It's now a fact -- the Afghans are doing the bulk of the fighting," he told a hearing in Washington.

President Hamid Karzai on Wednesday made a rare public criticism of national security personnel during a speech at the opening of a new session of parliament.

"While I fight everyday, without any exception, with the foreigners for the human rights of the Afghan people... we learn that our own security forces violate the rights of the Afghan people," he said. "This is unforgivable."

A UN investigation earlier this year found evidence of torture being used by police and the intelligence agencies in Afghan prisons.

The government at first rejected the report but later confirmed some of its findings.