Kabul: Heavily armed troops and police patrolled tense Kabul streets on Friday in a nervous wait for the main Muslim prayers, after a deadly week of anti-US unrest over the burning of Qurans at a military base.

The government called for calm to prevent Taliban insurgents from capitalising on fury over the desecration of Islam's holy book, but the response may hinge on how the mullahs treat the issue in their Friday sermons.
 Three days of bloody demonstrations have killed 14 people, including two American soldiers, forcing US President Barack Obama to apologise over the Quran burning at the Bagram airbase north of Kabul.

The US embassy warned that any new protests on Friday could turn violent and target Westerners, urging Americans in Afghanistan to "shelter in place and avoid any unnecessary movement".

The embassy has been in lockdown for days, while extra security forces are protecting foreign missions and other strategic places, some armed with rocket-propelled grenades and heavy machine-guns.

At least one demonstration was reportedly planned for Kabul and more were possible after the main weekly Muslim prayers, the embassy said.

An Afghan government delegation investigating the burning of the Qurans described it as a "shameful incident" but issued an overnight call for calm in a country wracked by 10 years of war against hardline Islamist insurgents.

"In view of the particular security situation in the country, we call on all our Muslim citizens of Afghanistan to exercise self-restraint and extra vigilance in dealing with the issue," the delegation said in a statement.

It urged Afghans to "avoid resorting to protests and demonstrations that may provide ground for the enemy to take advantage of the situation" and noted that the United States had pledged that those responsible would be held accountable.

Obama's apology came in for strong criticism back home, as competition builds in a presidential election year.