KABUL, Afghanistan — An Afghan soldier opened fire on U.S. troops in a restive eastern province of Afghanistan, killing three and injuring one, the Pentagon said Saturday.

The shooting occurred in the Achin district of Afghanistan’s Nangarhar province, according to a U.S. defense official, an area where the Islamic State and Taliban insurgents are contesting territory.

Attahullah Khogyani, spokesman for the provincial governor in Nangarhar province, said the Afghan soldier was killed after the attack.

The Taliban claimed responsibility for the attack. Taliban spokesman Zabihullah Mujahid said that a Taliban loyalist had infiltrated the Afghan army “just to attack foreign forces.”

The Pentagon said the incident is under investigation.

White House spokesman Raj Shah told reporters traveling with President Donald Trump in New Jersey that Trump was “following the emerging situation in Afghanistan.”

The U.S. deaths come as the Trump administration is weighing whether to send more U.S. troops to Afghanistan.

Achin has been the site of heavy fighting in recent months as U.S. forces, including Army Rangers and Green Berets, have been working alongside Afghan commandos to route the Islamic State from the area. Three U.S. soldiers had died there this year before Saturday’s shooting.

The Taliban are also active in the area, and there have been reports of clashes between the insurgent groups in recent weeks.

Allied commanders provided limited details.

“We are aware of an incident in Eastern Afghanistan. We will release more information when appropriate,” Douglas High, the public affairs officer for Operation Resolute Support, the NATO mission in Afghanistan, said in an email statement.

Also Saturday, two Afghan border policemen were killed by U.S. aircraft fire during a joint operation in the southern province of Helmand.

The U.S. military in a statement apologized for the deaths and said the incident was under investigation.

The three previous U.S. soldiers’ deaths in Achin make up the entirety of U.S. combat fatalities in Afghanistan in 2017.

In early April, Staff Sgt. Mark De Alencar, 37, was killed by small-arms fire, followed by Sgt. Joshua Rodgers, 22, and Sgt. Cameron Thomas, 23, at the end of the month.

Rodgers and Thomas were killed during a joint Afghan-U.S. nighttime raid on an Islamic State headquarters building.

The Pentagon is investigating if they were mortally wounded by friendly fire. The raid resulted in the death of the emir of the Islamic State’s branch in Afghanistan, Abdul Hasib, according to the Pentagon and Afghan officials. More than 30 other militants were killed.

Achin was the also site where U.S. Special Operations troops in April dropped the GBU-43, a 22,000-pound bomb known as the MOAB, on a purported cave complex where insurgents were believed to be hiding. The blast flattened a swath of the countryside. While Afghan officials said dozens of militants were killed, the Pentagon has remained mum on what the bomb accomplished.

The Afghan branch of the Islamic State, known as ISIS-K, is made up of militants pulled from other groups and has turned into one of the main counterterrorism efforts for the United States in Afghanistan. Although military officials say the group is far smaller than it was at its height in 2015, an estimated 600 to 800 militants, located mainly in remote mountainous areas, continue to pose a threat to U.S. and Afghan troops.

Associated Press contributed.