Two United States service members were killed during a joint operation with Afghan forces in northern Afghanistan on Friday, bringing to four the American combat deaths in Afghanistan so far this year, Afghan and American officials said.

A spokeswoman for the American-led NATO mission in Afghanistan confirmed the two deaths in a brief statement, but it did not release any details of the attack.

“The incident is under investigation and we have no additional information to provide,” said the spokeswoman, Sgt. 1st Class Debra Richardson.

Read more: New York Times