An Afghan soldier shot dead three US troops Saturday, officials said, in the latest "insider attack" to shake efforts by the two armies to work together to defeat Taliban militants.

Attacks in which Afghan forces turn their guns on their international partners have killed scores of US-led troops, breeding mistrust and undermining efforts to train up local forces ahead of the NATO coalition's withdrawal next year.

"Three International Security Assistance Force (ISAF) service members died when an individual wearing an Afghan National Security Forces uniform shot them in eastern Afghanistan today," a statement from the coalition said.

A US defence official confirmed to AFP that the three victims were from the United States.

An Afghan official, speaking on condition of anonymity, told AFP that the attack happened during a training session in the insurgency-hit province of Paktia.

The Afghan soldier opened fire on US soldiers, killing two on the spot, he said. A third later died of his wounds.

The attacker was killed when Americans and Afghan soldiers returned fire.

There have been seven "insider attacks" this year against coalition forces, compared with 48 in 2012. ISAF officials say the decline has been due to better vetting, counter-intelligence and cultural awareness.

Foreign soldiers working with Afghan forces are regularly watched over by so-called "guardian angel" troops to provide protection from their supposed allies.

ISAF officials say that most insider attacks stem from personal grudges and cultural misunderstandings rather than Taliban insurgent plots.

Afghan soldiers and police are taking on responsibility for battling the militants from 87,000 NATO combat troops who will leave by the end of 2014 -- 13 years after a US-led invasion brought down the Taliban regime.

But the 350,000-strong security forces are suffering a steep rise in casualties as the NATO combat mission winds down and Afghan authorities try to bring stability ahead of April's presidential election.

On Friday, Afghanistan's interior ministry confirmed that 18 policemen had been killed in a Taliban ambush in the northeastern province of Badakhshan.

The latest incidents will heighten concerns that Afghan forces cannot provide effective security across the country, where a US-led invasion ousted the hardline Taliban regime in 2001.

Also on Saturday, Pakistan said it had released from prison Abdul Ghani Baradar, who is often described as the former Taliban second-in-command, and the most high-profile militant commander detained in Pakistan.

The Afghan government, which had long called on Islamabad to free Baradar, hopes that the release will boost efforts to reach a peace settlement with the Taliban.

"Baradar is someone who has always been eager to join peace negotiations, and we hope he joins peace talks soon. We are optimistic about it. He is still an influential figure, and the Taliban still respect him," Mohammad Esmail Qasimyar, senior member of the Afghan High Peace Council, told AFP.

Baradar's release brings to 34 the number of Taliban detainees that Pakistan has freed since last year.

But analysts say that there is little evidence that the release of any of the Taliban detainees has had a positive effect on peace efforts, and several are understood to have returned to the battlefield.

Political analyst Talat Masood said the announcement was a "sort of a confidence-building measure between Pakistan and Afghanistan".

"However, this release is not likely to make any significant difference in the negotiating process," he said.