Anti-US demonstrations erupted in Kabul and across Afghanistan on Friday in a fourth day of protests over the burning of the Koran at an US-run base, police and witnesses said.

The crowds shouted "death to America" and security forces fired into the air in Kabul, where at least one demonstrator was shot in the hip, an AFP reporter said.

"We have demos in five locations in Kabul," senior police official Mohammad Zahir told AFP. "We have no violence so far."

Other protests broke out in northern Baghlan and Kunduz provinces, as well in central Bamiyan and Ghazni and eastern Nangarhar, AFP correspondents said.

The demonstrations erupted after the main weekly Muslim prayers with at least one mullah in Kabul urging the faithful to take to the streets over the desecration of Islam's holy book.

Bloody demonstrations since Tuesday have killed 15 people, including two American soldiers, forcing US President Barack Obama to apologise over the Koran burning at the Bagram airbase north of Kabul.

The government had called for calm to prevent Taliban insurgents from capitalising on fury over the incident, while the US embassy warned that protests could turn violent and target Westerners.