Media playback is unsupported on your device Media caption Afghan police and plain-clothed security officials fire shots into a crowd of about 500 protesters in Kabul

At least another eight people have been killed, including two Nato soldiers, in violence across Afghanistan, after the burning of the Koran at a US base.

Many others were injured in the protests, while armed men also attacked at least two military installations.

US President Barack Obama has sent his Afghan counterpart Hamid Karzai a letter of apology for the burnings.

Crowds shouting "death to Obama" have been throwing stones and setting fire to the US flag.

Mr Obama's letter, delivered by the US ambassador to Afghanistan, assured the Afghan president that US authorities would question all those responsible.

"I convey my deep sympathies and ask you and the people to accept my deep apologies," the letter said.

Meanwhile the Taliban has called on Afghans to kill and beat all invading forces in revenge for "insulting" the Koran.

In a statement a Taliban spokesman said Afghans should "not stop at protesting" but instead target military bases and personnel to "teach them a lesson that they will never again dare to insult the Holy Koran".

On Wednesday seven people were killed and dozens injured in protests over the burning of the copies.

'Death to America'

President Karzai met tribal leaders and politicians in an effort to find ways to calm tension.

At the scene It started at about 09:00 when people from different villages around Baghlan converged on the town centre. About 1,000 demonstrators gathered in front of the police station and there was a lot of anger and violence. Then suddenly we heard an outbreak of machine-gun fire. We went to the hospital where the injured were taken and a wounded policeman there told us that demonstrators shot at police. Officials say they are conducting an investigation to find out who opened fire. After the violence, people escaped from the area, shops were closed and eventually demonstrators left. But it was an intense episode. People were shouting anti-American slogans expressing their outrage at the burning of the Koran. They also accused the Americans of being opposed to their religion.

Afghan security officials fear the protests could spread further, with pressure on people in other towns and cities to show their outrage at the desecration.

The BBC's Andrew North, in the Afghan capital, says many officials sympathise with the outrage the US has provoked across the country.

He says Friday prayers may spark more tensions, depending on the tone set by religious leaders.

So far police, local officials and tribal elders have told the BBC there have been major protests in at least nine areas across the country, each involving many hundreds of people.

The worst incident was in Khogyani in Nangarhar province, where a man wearing an Afghan army uniform killed two Nato soldiers who are believed to be from the US.

Two protesters were also killed and seven injured as Nato forces opened fire when armed men attacked the US/Afghan base.

Further south, in Uruzgan province, two people were killed and at least eight others wounded, three of them police, in clashes between protesters and Afghan security forces, local officials told the BBC's Bilal Sarwary.

They said demonstrators were carrying guns, metal bars and sticks.

In northern Baghlan province one civilian was killed and two others injured, while two police were also hurt.

Image caption Afghan soldiers confiscated American flags during the protests in Kabul

Another person was killed in Laghman province east of Kabul, where local police said several hundred people were chanting "Death to America".

More than 3,000 people gathered in Mehtar Lam, the capital of Laghman province, with some burning an effigy of President Barack Obama.

Police say fights broke out as they stopped hundreds of protesters entering the centre of Kabul.

And in Asadabad, around 1,500 demonstrators were said to be burning US flags and tyres and shouting anti-American slogans.

A French military base to the east of Kabul was also attacked.

Earlier President Karzai said that "Afghan security forces should not use violence... and [should] protect civilian lives and property".

In a statement after an emergency debate, Afghan MPs condemned what had happened.

They also called for punishment of those responsible and asked the Afghan government to send its own delegation to Bagram to establish exactly what happened and why.

Isaf investigation

Muslims consider the Koran the literal word of God and treat each book with deep reverence.

The Nato-led Isaf force is now investigating the incident, a spokesman told the BBC.

"It was the local workers who discovered the nature of the material and therefore stopped worse things from happening," said Brig Gen Carsten Jacobson.

Last year, at least 24 people died in protests across Afghanistan after a hardline US pastor burned a Koran in Florida.