Afghan officials said Friday that at least seven people were killed in protests around the country against the burning of Qur'ans at a U.S. air base, bringing the overall death toll to 20 after four days of demonstrations.

The fresh protests were evidence that President Barack Obama's apology has not calmed Afghans enraged by the incident at Bagram Air Field earlier this week.

Afghan President Hamid Karzai, parliamentarians and some clerics have also called for an end to the protests until an investigation into the incident is concluded in coming days.

Six dead and 35 injured in Herat

The governor's office in western Herat province said six died in three incidents there.

Muhiuddin Noori, a spokesman for the governor, said three people were killed when a truck full of ammunition exploded after protesters set it ablaze. Three others died in two separate incidents when armed men among the protesters exchanged gunfire with security forces. He said at least 65 people were injured in the three protests.

In northern Baghlan province, Governor Abdul Majid said another protester died when Afghan security forces fired in the air to prevent demonstrators from storming a Hungarian base.

"There was a peaceful protest, but when it ended about 200 irresponsible young people ran toward the base and tried to enter the gate. There was shooting from the Afghan police and the army from several places and one man died and three were wounded," he said.

Protests follow deaths of two U.S. soldiers

The violence came as the top U.S. commander in Afghanistan has told his troops that "now is not the time for revenge" for the deaths of two U.S. soldiers killed in Thursday's riots.

He told them Thursday to resist whatever urge they might have to strike back after an Afghan soldier killed the two American troops. His comments were released Friday.

"There will be moments like this when you're searching for the meaning of this loss. There will be moment like this, when your emotions are governed by anger and a desire to strike back," Allen said. "Now is not the time for revenge, now is not the time for vengeance, now is the time to look deep inside your souls, remember your mission, remember your discipline, remember who you are."

Afghan National Army Gen. Sher Mohammed Karimi, who travelled to the base with Allen, told the U.S. troops that their sacrifice is not wasted.

Karimi says the Americans and Afghans together are "fighting an enemy of humanity."

Qur'ans found in pit

The unrest started Tuesday, when Afghan workers at the sprawling Bagram air base noticed that Qur'ans and other Islamic texts were in the trash that coalition troops dumped into a pit where garbage is burned. Some Afghan workers burned their fingers as they tried to salvage some of the books. Afghan government officials said initial reports indicated four Qur'ans were burned.

The materials had been taken from a library at Parwan Detention Facility, which adjoins the base, because they contained extremist messages or inscriptions. Writing inside a Qur'an is forbidden in the Islamic faith, although it was unclear whether the handwritten messages were found in the holy book or other reading materials.

A military official said it appeared that detainees at the prison were exchanging messages by making notations in the texts.

Obama apology

Obama apologized Thursday. In a letter to Karzai, Obama expressed "regret and apologies over the incident in which religious materials were unintentionally mishandled."

But that apology held little sway with Afghans upset at what they considered a serious violation of their religion.

"We don't care about Obama's apology," said Kamaluddin, a 25-year-old Kabul protester who uses only one name. "We have to protest to be responsible to our God. They are burning our Qur'an. An apology is not enough."