Two American soldiers and at least 15 Afghans have been killed as crowds besieged US and NATO bases for a third day Thursday in an escalating protest over US troops burning copies of the Koran.

The three days of protests have seen violent clashes across Afghanistan, with crowds in a number of areas blocking highways and attacking and burning government facilities and, in the eastern city of Jalalabad, NATO fuel trucks.

The two American soldiers were shot to death Thursday—and four others were wounded—in eastern Nangarhar province at a military base that crowds of outraged Afghans attempted to storm. A uniformed Afghan soldier reportedly opened fire on the American troops and then fled, disappearing into the mass of protesters.

At least two Afghan civilians were killed outside the same base when security forces fired upon protesters to keep them from forcing their way into the facility. Two others, including a 12-year-old Afghan boy, were killed in the Batikot district of the same province, Afghanistan’s Pajhwok news service reported.

The killing of the US troops came on the heels of a public appeal by the Afghan Taliban to Afghan troops and police to attack foreign occupation forces. The statement called on “all the youth present in the security apparatus of the Kabul regime to fulfill their religious and national duty… by turning their guns on the foreign infidel invaders.”

In neighboring Laghman province, at least one civilian was shot dead when Afghan troops opened fire to keep stone-throwing protesters from overrunning their base.

In the central Afghan province of Uruzgan, at least three protesters were killed and 12 others wounded in what Afghan authorities described as “crossfire” between security forces and armed resistance fighters who had joined the demonstration.

And in northern Baghlan province, a policeman was killed after demonstrators were fired upon and responded by attacking the police with stones and sticks.

At these and other locations, demonstrators took up the slogans of “Death to America” and “Down with Karzai,” referring to the president of the US-backed regime in Kabul, Hamid Karzai.

At least eight Afghans were killed during demonstrations that swept the country on Wednesday. Scores of others were wounded or injured on both days.

There are fears within the occupation forces and the Karzai regime that Friday will see even greater eruptions after prayer services in the country’s mosques. The US Embassy in Kabul and other US bases and facilities have been placed on lockdown, with no one allowed in or out.

The spark that ignited this nationwide conflagration was the report that Afghan workers at an incinerator pit at the US Bagram Air Base found American troops dumping copies of the Koran and other Islamic religious materials in a pile of debris for burning.

The material had been seized from a library at the Bagram prison, where the American occupation forces hold Afghans suspected of being members of the Taliban and other resistance forces. A military official told CNN that they were taken for burning because they were believed to contain “extremist inscriptions” and supposedly had been used to “facilitate extremist communications.”

The Obama administration and the Pentagon have attempted to defuse the crisis, with a presidential apology and an announcement by a top military commander that all US forces in Afghanistan will undergo training in the proper handling of Korans and other religious material.

“I wish to express my deep regret for the reported incident. I extend to you and the Afghan people my sincere apologies,” Obama wrote in a letter presented to Karzai Thursday by US Ambassador Ryan Crocker. “The error was inadvertent; I assure you that we will take the appropriate steps to avoid any recurrence, to include holding accountable those responsible.”

For masses of ordinary Afghans, however, such apologies are worse than meaningless. The latest Koran burning is seen only as one more episode in the country’s oppression and humiliation at the hands of the US-led occupation. It follows the release just last month of a video showing American Marines urinating on the corpses of slain Afghans, which also provoked protests. Reports of desecrations of the Koran have provoked at least two previous waves of deadly protests.

“This is not just about dishonoring the Koran, it is about disrespecting our dead and killing our children,” Maruf Hotak, 60, who joined a protest on the outskirts of Kabul, told the New York Times. “They always admit their mistakes. They burn our Koran and then they apologize. You can’t just disrespect our holy book and kill our innocent children and make a small apology.”

On Wednesday, in the midst of the spreading protests, officials in Nangarhar province reported that a NATO helicopter attacked a school. “This morning a school was attacked by a NATO helicopter. Nine children, all girls, and the school's janitor have been injured,” a spokesman for the provincial government said. At least five of the schoolgirls remained hospitalized.

The attack followed the admission by NATO last week that a February 8 air strike had killed eight children in northeastern Kapisa province.

In addition to these constant killings, deteriorating social conditions in Afghanistan are also fueling the mass resentment and anger that have exploded in the Koran demonstrations.

On Thursday, Amnesty International issued a report detailing the abysmal situation confronting some half million Afghans who have been forced from their homes by the US-led war and are struggling to survive in urban slums.

“Thousands of people are finding themselves living in freezing, cramped conditions and on the brink of starvation, while the Afghan government is not only looking the other way but even preventing help from reaching them,” said Horia Mosadiq, Amnesty International’s Afghanistan researcher. Twenty-eight children have either frozen or starved to death in the makeshift camps around Kabul in recent weeks.

The nationwide protests have come at a most inopportune time for the Obama administration, which is attempting to negotiate a pact with the Karzai regime allowing US troops to remain in Afghanistan long after the formal NATO withdrawal deadline at the end of 2014.

According to the Associated Press, Washington and the Karzai regime had been considering a deal that would separate negotiations on the disputed issues of US special forces night raids on Afghan homes and the turning over of American-run prisons to Afghan authorities in order to sign a broad agreement granting the US military permission to maintain bases in the country.

Karzai has demanded that the night raids—which have provoked intense anger against the occupation and its puppet regime—either cease or be placed under Afghan leadership. The US military command has flatly refused to consider either option and is planning to rely ever more heavily on special operations units as well as aerial bombardment as regular forces are withdrawn from the country.

US officials have also claimed that Afghans are not capable of taking over the US-run prisons, where detainees have complained of torture. Officials in the Afghan regime have insisted that if they had been running the facility at Bagram, the present crisis over the Koran burnings would never have happened.

While Karzai and his cohorts are anxious to end some of the most egregious practices of the US-led occupation, for fear that the popular outrage will only strengthen the Afghan resistance, at the same time they recognize that their regime would be overthrown in short order if all US forces were to leave the country.

Both Washington and the Karzai regime have indicated that they want to sign a pact on the continued US military presence in Afghanistan before a NATO summit that will be held in Chicago in May.