Kabul: Over 100 people have died in violence that was sparked by protests against a US pastor's burning of the holy Quran in Afghanistan's southern city of Kandahar, officials said.

Police said the protesters have been joined by Taliban militants and witnesses said that police had opened fire on a crowd that tried to storm a UN office located next to a government building.

Protests over the public burning of the Quran in the US grew into riots on Friday.

Violence started in the northern city of Mazar-i-Sharif, where seven UN workers and four protesters died as a mob stormed a UN office. It moved to the southern city of Kandahar on Saturday, continuing on Sunday. Violence has now spread to Jalalabad in the east, where at least 20 people have been killed.

US President Barack Obama termed the killings ‘outrageous’.

"The desecration of any holy text, including the Quran, is an act of extreme intolerance and bigotry. However, to attack and kill innocent people in response is outrageous, and an affront to human decency and dignity," Obama said in a statement released by the White House.

Russia condemned the "unacceptable" attack on the UN mission in Mazar-i-Sharif and demanded that measures be taken to stop violence against UN staff.