Five U.S. special operations troops were wounded in combat with Islamic State fighters in eastern Afghanistan, the senior U.S. commander in the country said Thursday.

It appeared to be the first reported instance of U.S. troops being wounded in fighting against the Islamic State in Afghanistan. U.S. military spokesmen in Kabul said they were researching the question of whether there have been previous casualties in combat with IS, which is present mainly in the country's eastern regions. IS bases in the eastern province of Nangarhar, which borders Pakistan, are currently being targeted by an Afghan military offensive, backed by U.S. troops.

The Afghan offensive began on Saturday, hours after the IS group claimed responsibility for a suicide bomb attack in the capital Kabul that killed around 80 people.

Gen. John Nicholson, the top commander of U.S. and NATO forces in Afghanistan, said the five wounded Americans were hit by small arms fire or shrapnel during a combat operation conducted with Afghan special operations forces to clear areas once controlled by the Islamic State in Nangarhar.

He did not say exactly when the injuries happened. The Pentagon later issued what it called a clarification, saying one of the five was wounded on Sunday and the other four on Monday.

"There was not one incident or specific firefight, but these service members were wounded over the course of the clearing operations General Nicholson described," a Pentagon spokesman, Adam Stump, said in a statement. "As General Nicholson indicated, we're not able to discuss further specifics at this point of the counterterrorism operation."

Speaking to reporters at the Pentagon from his headquarters in Kabul, Nicholson said none of the wounds are life-threatening. Three of the soldiers have been evacuated to a U.S. military hospital in Germany, he said. "They're in good spirits," Nicholson said. "They've talked to their families. We expect a full recovery."

The other two wounded have been returned to duty in Afghanistan, he said.

Nicholson said the casualties happened during a counterterrorism operation in which Afghan forces have recaptured ground previously held by the Islamic State, following U.S. airstrikes.

Thus far, operations have been successful, Nicholson said.

"We have helped the Afghan security forces to reclaim significant portions of the territory that was previously controlled by Daesh," he said, using an alternative acronym for the Islamic militant group. "We have killed many Daesh commanders and soldiers, destroyed key infrastructure capabilities, logistical nodes, and Daesh fighters are retreating south into the mountains of southern Nangarhar as we speak."

He said the number of IS fighters in Afghanistan has declined from an estimated 3,000 at the start of this year to between 1,000 and 1,500. The majority of those in Nangarhar are former members of a Pakistan Taliban group known as TTP, he said, adding that they were largely forced out of Pakistan by a government offensive and joined IS earlier this year.