KABUL, Afghanistan (CNN) — Afghan rage over the burning of Qurans by NATO troops showed little signs of subsiding Thursday even after a President Barak Obama apologized for the “error.”

In a letter delivered to Afghan President Hamid Karzai, Obama called the act “inadvertent,” Karzai’s office and National Security Council spokesman Tommy Vietor said Thursday.

“We will take the appropriate steps to avoid any recurrence, including holding accountable those responsible,” Obama said in the letter delivered by Ryan Crocker, the U.S. ambassador to Afghanistan.

Afghanistan erupted in violent demonstrations since the troops’ burning of the Islamic religious material at the beginning of the week. Many feared further unrest Friday, the Islamic holy day.

Two American troops were killed Thursday by a man wearing an Afghan National Army uniform, a U.S. official said, asking not to be named discussing casualties.

The gunman is thought to have been acting in conjunction with a protest taking place outside the base, the official said.

The protest and shootings came as the Taliban called on Muslims to attack NATO military bases and convoys and kill soldiers following the admission that NATO troops had incinerated the religious material at Bagram Airfield.

Afghan officials investigating urged Afghans to “exercise self-restraint” and “avoid resorting to protests and demonstrations that may provide ground for the enemy to take advantage of the situation.”

The investigators called the burning of religious material “insulting and shameful,” saying NATO officials had promised to bring the “perpetrators of the crime … to justice as soon as possible.”

Muslims believe the Quran is the word of God, so holy that people should wash their hands before even touching the sacred book. Desecrating the Quran is seen as an affront — an act of intolerance and bigotry.

At least two demonstrators also were killed in the exchange of gunfire near the base where the two Americans died, said Haji Mohammad Hassan, chief of Khugyani district in eastern Nangarhar province.

Two U.S. soldiers and seven demonstrators were injured in the clash, too, he said.

“We don’t know who started the shooting first and what kind of guns were used, but we have started our investigation to find out the details of the incident,” Hassan told CNN.

He said there were 200 to 300 demonstrators but the protest was now under control.

There was also a larger demonstration near a Norwegian-run Provincial Reconstruction Team compound in Maimana, the provincial capital of Faryab province, a regional police spokesman said.

Afghan security forces prevented the 700 to 800 demonstrators from getting into the compound, said police spokesman Lal Mohammad Ahmadzai said.

They burned a few civilian vehicles parked near the compound, he said.

American diplomats in Kabul and the north and south of Afghanistan are on lockdown for a second day in the face of protests, U.S. Embassy spokesman Mark Thornburg said.

At least five people were killed in demonstrations Wednesday.

The commander of NATO’s International Security Assistance Force, Gen. John Allen, apologized Tuesday for the incineration of the materials and said it was a mistake. The materials were gathered for disposal and were inadvertently given to troops for burning, Allen said.

He said: “It was not a decision that was made with respect to the faith of Islam. It was a mistake. It was an error. The moment we found out about it, we immediately stopped and we intervened.”

The Taliban on Thursday rejected the apology.

In an e-mail message, the Islamist militia accused “the invading infidel authorities” of trying to calm the situation with two “so-called show(s) of apologies, but in reality they let their inhuman soldiers insult our holy book.”

They called on Afghans to take revenge “until the doers of such inhumane actions are prosecuted and punished.”

“We should attack their military bases, their military convoys, we should kill their soldiers, arrest their invading soldiers, beat them up and give a kind of lesson to them that they never dare to insult the holy Quran,” the message said.

Afghan religious scholar Anayatullah Baligh said it can be appropriate to burn a damaged Quran to dispose of it, but that it should be done by a Muslim performing the act respectfully.

“I can’t tell you whether Americans intentionally burned the copies of the holy Quran to make Muslims angry or if they did it mistakenly,” he said, but said their “carelessness” was “a crime they have committed against the holiest book of 2 billion Muslims around the world.”

A military official told CNN on Thursday that it was unclear at this point how many Qurans were involved in the improper disposal and accepted that some had been partially burned.

American troops at the base would not have been able to read the texts and that would have contributed to the mistake, the official said, asking not to be named discussing an ongoing investigation.

A second military official said earlier the materials were removed from a detainee center’s library because they had “extremist inscriptions” on them and there was “an appearance that these documents were being used to facilitate extremist communications.”

But U.S. apologies have not been enough to appease angry Afghans, who massed outside the Bagram base Tuesday, chanting “Death to America! Death to the Afghan government! Long live Islam!”

Protesters burned tires and threw rocks Wednesday outside Camp Phoenix near Kabul International Airport, the U.S. Embassy said in its official Twitter feed. It asked Americans to avoid the area, saying the protests had “turned violent.” It also suspended all travel.

In Jalalabad, hundreds chanted “Down With America” as crowds gathered near the airport.

Authorities have questioned some troops as part of their investigation, but no one has been detained, a coalition official said.

Last year, when controversial Florida pastor Terry Jones presided over what he called a trial of the Quran and burned a copy, Afghans took to the streets. In the northern Afghan city of Mazar-e Sharif, demonstrators stormed a U.N. office and killed 12 people. In Kandahar, three people were killed, and nine in another when police and stone-throwing demonstrators clashed.

American officials vociferously condemned the pastor’s act.

In 2010, Afghans protested outside the Forward Operating Base Mirwais in response to an alleged Quran burning inside the base. But coalition forces called the suspected burning a routine burn-pit session in which military documents were destroyed.

— CNN’s Lesa Jansen in Washington contributed to this report.