Image caption Patrols conducted jointly between Afghan and Nato-led forces will no longer happen routinely

Nato says it is restricting operations with Afghan troops following a string of deadly attacks on its personnel by rogue Afghan security forces.

Only large operations will now be conducted jointly, with joint patrols evaluated on a case-by-case basis.

Nato said these were "prudent, but temporary, measures to reduce our profile and vulnerability".

Nato commanders have been frustrated that the Afghans have not done more to stem the rise in attacks, analysts say.

UK Defence Secretary Phillip Hammond said the announcement did not amount to a change in strategy in the Nato-led International Security Assistance Force (Isaf) but was to "ensure that any partnering with Afghan troops at lower than battalion level is properly approved with proper risk assessments in place".

The move came as a suicide bomber targeted a bus carrying foreigners in the capital, killing 12 people.

The attack, on Tuesday morning, happened on a major road leading to the international airport and reports suggest those on board worked at the airport.

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 which has sparked protests in countries across the Middle East, Africa and Asia.

Media playback is unsupported on your device Media caption Nato's Anders Fogh Rasmussen: "Our partnership with the Afghan forces continues"

Meanwhile 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.

Rogue 'surge'

Isaf's joint command said "events outside of and inside Afghanistan" related to the anti-Islam film were part of the reason for its restrictions on joint operations.

Afghanistan has seen days of often violent protests over the video, which was made in the US.

On Monday, hundreds of protesters threw stones and torched police vehicles in an angry protest against the film in Kabul. There were reports on Tuesday of a new protest in the northern city of Kunduz.

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

Analysis 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 said he was concerned about the effect of insider attacks, but insisted they did not mean the Taliban was getting stronger or regaining lost territory.

Afghan 'green-on-blue' attacks 2007 - 2 attacks, 2 Isaf soldiers dead

2008 - 2 attacks, 2 dead

2009 - 6 attacks, 10 dead

2010 - 6 attacks, 20 dead

2011 - 21 attacks, 35 dead

2012 (so far) - 36 attacks, 51 dead Source: International Security Assistance Force (Isaf)

The US would do all it could to minimise risks to its forces, he said, 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".

But the BBC's Bilal Sawary in Kabul says this will be a huge morale blow to the Afghan personnel, who often rely on the better-equipped and more experienced Isaf troops to come to their aid when they are under attack.

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

It will also be a propaganda victory by the Taliban, says our correspondent, with the implication that Nato does not trust its Afghan partners.

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

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

Image caption UK soldiers Sgt Gareth Thursby (left) and Pte Thomas Wroe died in an insider attack on Saturday

The Afghans say they are aware of the growing problem of insider attacks, but there has been enormous frustration among Nato commanders that Afghan officials have not been doing enough to prevent them.

Correspondents say the Isaf shift is clearly aimed at sending a signal to the Afghan government that it must improve its vetting of new entrants to the Afghan army and police force.

But with 7,000 new recruits a month joining the Afghan army alone, it is a huge challenge to ensure Taliban militants do not slip through the net, they say.