The violence began when demonstrators gathered at the Blue Mosque in the northern city of Mazar-i-Sharif after Friday prayers and marched toward the headquarters of the UN mission.

A small group reportedly broke away from the main crowd and overwhelmed security personnel at the compound, seizing weapons from the guards and firing on the embassy.

"This was an outrageous and cowardly attack against UN staff which cannot be justified under any circumstances and I condemn it in the strongest possible terms," UN Secretary General Ban Ki-moon said to reporters while visiting the Kenyan capital Nairobi.

Ban confirmed that UN employees had been killed, though he said "the details are not completely clear".

Reports are circulating that those killed included local and foreign employees, UN guards and Nepalese Gurkha soldiers hired to protect the building. Two of the foreign workers were beheaded while the others were shot.

At least five of those killed included locally hired Afghan security guards, police sources report. Bodies continue to be brought to the hospitals, suggesting that the death toll may rise.

Popular Afghan television channel Tolo TV reported that the top UN official in the city, Staffan de Mistura, was among the victims. But Mistura's actual condition has yet to be confirmed.

A Koran was burned by preacher Wayne Sapp in Florida after a mock trial was overseen by Pastor Terry Jones of Dove World Outreach Center in Gainesville, Florida. The church said on its website that the Koran would be “executed" if it was “found guilty of causing murder, rape and terrorism”.

Later on the website, the church posted that the “Koran was found guilty” during the mock trial and that a copy of the Islamic holy book was burned inside the building.

The following day, Afghan President Hamid Karzai called on the US government to prosecute those responsible for burning the Quran.

Jones had earlier grabbed international attention after threatening to burn the Quran to mark the 9/11 anniversary last year. Most American evangelicals did not share Jones' enthusiasm for burning the Quran.

"It sounds like the proposed Quran burning is rooted in revenge," National Association of Evangelicals President Leith Anderson had said in a statement responding to Jones' threats last year.

"Yet the Bible says that Christians should 'make sure that nobody pays back wrong for wrong, but always try to be kind to each other and to everyone else' (I Thessalonians 5:15)."

Amidst calls for restraint, including from US President Barack Obama, Jones relented last year but seems to have renegaded on his previous promise with the recent Koran burning in Florida.

"We only did it because we felt there needed to be an outcry against Islam, because Islam is presenting itself as a religion of peace," Jones had said to The Christian Post last year.

On Friday, an unrepentant Jones said he was "not responsible" for the killings in Afghanistan during an interview broadcasted live on BBC. "They [Afghan demonstrators] used the Koran burning as an excuse to promote their violent activities," Jones said.

The situation in Mazar-i-Sharif remains tenuous as Afghan authorities block roads leading in and out of the city. Special army and police units are currently being deployed to prevent further unrest in the area. Considered one of Afghanistan's safest cities, Mazar-i-Sharif was on the list of areas to be handed over to full Afghan security control later this year. The recent attack on the UN compound raises serious questions whether the city can make the planned transition.

Mazar-i-Sharif has been in the spotlight for the imprisonment of Afghan Christian convert Shoaib Assadullah, who was arrested in October last year. He had given a Bible to a friend, who later reported him to authorities. Assadullah was locked up in the city’s jail roughly the same time the international community rallied behind a second Afghan Christian convert, Said Musa, who was later released.

Friday’s attack is not the first time that a Koran-related controversy ignited violence amongst Afghanistan's devout Muslim followers.

In January last year, Afghan security forces shot and killed seven civilians after protesters attempted to overrun NATO bases and police facilities. The demonstrations were sparked by an alleged burning of the Koran by US soldiers in the country's southern province of Helmand, which is at the center of the insurgency.

Subsequent NATO and Afghan investigations found that no Koran had been burned by soldiers in any of the international military contingency in the region.

A previous protest in May 2008 turned deadly after one foreign soldier serving under NATO command was killed. Two Afghan protesters were also killed in the demonstration against the shooting of a Koran by a US soldier in Iraq.