The police officers had been told to be vigilant. They had been warned that protests could occur spontaneously and could again turn deadly, as they had for two days after U.S. military officials burned copies of the Koran.

But some of those same Afghan police officers showed few qualms in telling a foreign reporter that their mission left them deeply uneasy. What their government was asking, they said, was for police officers to quell protesters whose cause they fully shared.

“Afghans and the world’s Muslims should rise against the foreigners. We have no patience left,” said one police officer in central Kabul, who has worked at the same checkpoint since he joined the force seven months ago. He looked at his colleague, who stood next to him, nodding. “We both will attack the foreign military people.”

Police officers interviewed at four posts in the Afghan capital voiced the anti-American sentiments on Thursday, the same day that two U.S. troops in eastern Afghanistan were fatally shot by a man wearing an Afghan army uniform. The killings were the latest apparent incident of fratricide aimed at Americans within a nominally united U.S.-Afghan force, and they have added to misgivings among many U.S. troops about the loyalty of their Afghan counterparts.

In the wake of the Koran burning that came to light Tuesday at the largest U.S. military base in Afghanistan, some uniformed Afghan officers have worked tirelessly to keep the peace through three days of demonstrations and riots.

At least three protesters were killed Thursday, bringing the week’s death toll to 10, as some police officers were ordered to fire on demonstrators. More were injured Friday, and several were feared dead, in clashes that erupted when Afghans staged more demonstrations following the end-of-the week noon prayer services. A restive crowd in the western city of Herat converged near a U.S. military base, residents there said, and thousands gathered in eastern Kabul demanding retribution. About 100 protesters massed outside the Ministry of Defense in the capital, dispersing only when police fired into the air.

It has been Afghan civilians, not Taliban insurgents, who have taken the lead in the latest violence, and in five interviews, members of the Afghan police force made clear that they and others in positions of authority share in the anger and resentment.

“Those behind the act should be asked about their deed and must be punished,” said an officer near a U.S. military base in Kabul. “If I find the opportunity, I would shoot them in the head.”

The police officers would discuss their sentiments only on the condition of anonymity, saying they would risk their livelihoods if they were to sympathize publicly with those fomenting violence. But their comments left little doubt that the fallout over the U.S. military’s mishandling of the Korans includes fresh hostility among a crucial population of workaday Afghans, including some who man security checkpoints near Western installations.

U.S. apologies dismissed

In a bid to ease tensions, President Obama took the unusual step of extending a personal apology for the incident.

White House spokesman Jay Carney, speaking to reporters Thursday aboard Air Force One en route to Miami, said the apology was part of a three-page letter to President Hamid Karzai covering a variety of issues. Carney said the apology was “wholly appropriate, given the sensitivities to this issue,” as well as Obama’s “primary concern” for the safety of American military and civilian personnel in Afghanistan.

With tensions still high, however, the U.S. Embassy remained on lockdown for a second day and extended its travel restrictions to a typically peaceful part of northern Afghanistan.

In a meeting with Karzai early Friday, some members of the Afghan parliament demanded harsh retribution, while religious officials spoke of jihad and the urgent need to respond with violence. At the same time, the Taliban issued a harshly worded statement encouraging Afghan security officials to take up arms against Western forces.

On the streets of Kabul, police officers said they didn’t care about the flurry of U.S. apologies, including the one from Obama, or the demands of Afghan politicians. The offense felt was personal, most said, not diminished by contrition or inflamed by hostile rhetoric.

“It is difficult sometimes to convince people not to resort to protest,” said Qaseem Jangalbagh, the police chief of Panjshir province. Asked whether that included his own officers, he said, “It is a problem.”

Junior officers spoke more bluntly, saying they would shirk their duties rather than quash demonstrations and referring often to their own violent impulses.

“We should burn those foreigners,” said a police officer in his early 30s who has been in the force for almost 2 1 / 2 years. Like most of the country’s security officers, he was trained by NATO troops.

Anger within Bagram base

Police officers weren’t the only Afghans assumed to be U.S. allies who spoke of mounting friction. The first early morning protests Tuesday were led by Afghan employees of Bagram air base, where the religious materials were burned. NATO military officials have said in public statements that the incineration was accidental.

Some Bagram employees — who often face threats for aiding the United States — waved the charred books in the air, demanding a response.

Those employees, among the 5,000 Afghans who support the base’s operations, chanted “Death to America” and lobbed rocks at gates that some had entered for years. Some cursed their bosses and promised never to return to work at Bagram.

“How could we ever work for someone who could do this?” asked a 21-year-old man who said he had worked for two years in a warehouse on the base. “This couldn’t have happened by accident. This was meant to offend us.”

Taliban officials, who are in the middle of tenuous peace talks with the United States, had initially condemned the burning but stopped short of advocating violence — an uncharacteristically muted response. But in the written statement released Thursday, the insurgent group took a tougher stance.

The statement described the burning as a “deliberate” act, despite repeated statements by top U.S. officials that the books were sent to the incinerator by mistake. The Taliban statement said Afghans and Muslims should not be placated by the U.S. apologies and declared that protests and “mere slogans” were not enough of a response.

“For the defense of our holy book, we . . . must target the invaders’ military centers, their military convoys and their invading forces . . . so that they can never dare to desecrate the holy Koran again,” the statement said.

It was difficult to determine whether the attack Thursday on the U.S. troops was orchestrated by the Taliban or was a response to the group’s call for revenge. The attack occurred in eastern Afghanistan, the U.S. military said. Few details were released.

Special correspondent Sayed Salahuddin contributed to this report.