Turkish troops check the site of an attack in Bingol province

Militants have killed at least four Turkish soldiers and injured at least 50 in a rocket attack on a convoy in the south-east, security officials say.

Most of the injured were reportedly on a bus which caught fire when the convoy was hit on a highway connecting the provinces of Bingol and Mus.

The injured were rushed to local hospitals and sources warned the death toll might rise.

Security sources blamed the Kurdistan Workers' Party (PKK) for the attack, reports the BBC.

The Turkish army responded to the attack with an immediate operation backed by air power in the larger Bingol area, the Turkish news agency Anatolia reports.

Prime Minister Recep Tayyip Erdogan said this week that 500 Kurdish rebels had been "rendered ineffective" by his forces in the space of a month. The government often uses the phrase to mean killed.

He specified that 123 militants had been killed over the past 10 days near the south-eastern border with Iraq.

PKK attacks on Turkish targets recently escalated. Dozens of Turkish troops and civilians, including children, have been killed in recent bombings blamed on the group.

The PKK has been fighting for an ethnic homeland in south-eastern Turkey since 1984. It is classified as a terrorist organisation by Turkey, the US and EU. In all, more than 40,000 people have died in the conflict.

Afghan insurgent group Hezb-e-Islami has claimed it carried out the attack, which it says was in response to a recent anti-Islam video.

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Meanwhile NATO-led ISAF forces said they had arrested a Taliban leader and two insurgents they said were involved in last Friday's attack on the sprawling Camp Bastion in southern Helmand province.

The Taliban leader, said ISAF, was suspected of "providing support" to the militants who staged the audacious assault, which killed two US marines and destroyed six Harrier fighter jets.

The joint command of the NATO-led ISAF forces said "events outside of and inside Afghanistan" related to the anti-Islam film, which was made in the US, were part of the reason for its restrictions on joint operations.

Afghanistan - like many other countries in the Middle East, Africa and Asia - has seen days of protests over the video, some violent.

On Monday, hundreds of protesters threw rocks and torched police vehicles in an angry protest against the film in Kabul. The AFP news agency said hundreds more staged a new protest in the northern city of Kunduz on Tuesday.

Another prompt for the new restrictions is the recent surge in so-called "green-on-blue" attacks, ISAF said.

The shift in NATO's operational procedures has not been well explained. Considerable confusion remains as to what exactly will or will not change on the ground.

A NATO spokesman says that partnering operations below battalion level will have to be approved by a senior regional commander; the British defence secretary in contrast suggests most UK-Afghan operations will continue unchanged down to company level.

Clearly the aim is to reduce the exposure of NATO personnel to potential attack by uniformed Afghans.

The cumulative effect of these attacks strikes at the very core of NATO's mission.

With most NATO combat troops due to leave in 2014, operations are in transition between counter-insurgency and a training and mentoring role.

But training and mentoring require trust and a functioning relationship between NATO and Afghan personnel. It is this the so-called "green-on-blue" attacks destroys, and thus their significance goes well beyond the numerical count of the casualties they cause.

Fifty-one NATO troops have been killed by Afghan soldiers so far this year - 15 in August alone. In 2008, just two soldiers died in such attacks - though ISAF and Afghan force numbers have also increased substantially in that period.

Four US soldiers and two UK soldiers died in rogue attacks at the weekend. A fifth of UK soldiers killed this year in Afghanistan were killed not by insurgents, but by Afghan soldiers or police.

Joint operations will now only be conducted routinely at battalion level - large operations involving several hundred troops.

"This does not mean there will be no partnering below that level; the need for that will be evaluated on a case by case basis" but it will have to be approved by a two-star general, ISAF said.

It later clarified that the changes were temporary.

"In some local instances, operational tempo has been reduced, or force protection has been increased. These actions balance the tension of the recent video with force protection, while maintaining the momentum of the campaign," said a second statement.

NATO insisted it remained "absolutely committed to partnering with, training, advising and assisting our ANSF [Afghan National Security Forces] counterparts".

In a news conference, US Secretary of Defence Leon Panetta told reporters he was concerned about the effect of insider attacks.

But he insisted they did not mean the Taliban was getting stronger or regaining lost territory.

He said the US would do all it could to minimise risks to its forces, but "we will not lose sight of the fundamental mission here, which is to continue to proceed to assure a peaceful transition to Afghan security and governance".

Hammond said the changes would have "minimal impact" on UK operations. The UK has 9,500 troops in Afghanistan.

The BBC's Quentin Sommerville, in Kabul, says international and Afghan forces are meant to fight shoulder-to-shoulder against the Taliban and the new restrictions strike at the heart of NATO's strategy in Afghanistan.

In practical terms, US soldiers are already staying on their bases, while Afghans carry out patrols alone.

The Afghan ministry of defence said it had not been formally notified of the changes until a hurriedly convened meeting with NATO on Tuesday.