WASHINGTON (AP) — President Donald Trump is set to pay his respects on Wednesday at a Delaware military base when the remains of six U.S. service members killed in the crash of a refueling aircraft are returned to their families.

It will be the second time since launching military operations against Iran that the Republican president will attend the solemn military ritual known as a dignified transfer, which he once described as the 'toughest thing' he has had to do as commander in chief.

All six crew members of a KC-135 Air Force refueling aircraft were killed last week in a plane crash over friendly territory in Iraq while supporting operations against Iran. They hailed from states including Alabama, Indiana, Kentucky, Ohio, and Washington.

The crash raised the U.S. death toll in Operation Epic Fury to at least 13 service members. Around 200 U.S. personnel have been injured, with 10 sustaining severe injuries, according to the Pentagon.

Trump last traveled to Dover Air Force Base on March 7 for the dignified transfer of six service members who were killed by a drone strike in Kuwait. He saluted as flag-draped transfer cases containing the remains of the fallen service members were carried from military aircraft to vehicles awaiting their journey to the base mortuary for final arrangements.

“It’s the bad part of war,” he told reporters after the ceremony. When asked about his concerns regarding future transfers, he stated, “I’m sure. I hate to do it, but it’s a part of war, isn’t it?”

The crash followed an unspecified incident involving two aircraft in 'friendly airspace' over Iraq, but it was confirmed that the loss during a combat mission was not due to hostile or friendly fire, according to U.S. Central Command.

The Pentagon has stated that investigations are ongoing regarding the incident. Among those killed were three people from the 6th Air Refueling Wing at MacDill Air Force Base in Florida and three others from the 121st Air Refueling Wing at Rickenbacker Air National Guard Base in Ohio.