Two members of an NPR news crew, David Gilkey and Zabihullah Tamanna, were killed on Sunday while traveling in southern Afghanistan.

"They were traveling with an Afghan army unit when the convoy came under fire. Their vehicle was struck by shell fire," according to a statement by NPR.

Two other NPR crew members, correspondent Tom Bowman and producer Monika Evstatieva, "were in a following vehicle," NPR head of news Michael Oreskes told CNN. "Tom and Monika were not hurt."

Sunday's attack marks the first time in the 46-year history of NPR that one of its journalists has been killed on assignment.

Gilkey, 50, was an award-winning staff photographer and video editor for NPR. In the 15 years since the 9/11 attacks, he returned time and time again to Afghanistan and other conflict zones.

"David was profoundly committed to coverage of both Afghanistan and Iraq," Oreskes said. "He wanted to know what was happening to the people there. I think that's why he kept going back -- because he wanted to understand what was happening to the soldiers and civilians."

Tamanna was an Afghan freelance journalist hired by NPR to be a translator for its team of journalists. His Twitter profile also identified him as a freelancer with Anadolu News Agency.

In a telephone interview, Oreskes noted that "the Afghan journalists have been the bravest of all," documenting the ongoing conflict in the country while foreign correspondents rotate in and out.

The NPR team had been on assignment in the country for about three weeks. It was expected to be a month-long trip. The team had already filed multiple stories, including an in-depth report about an Afghan commando mission and a Memorial Day remembrance from Kandahar. Gilkey's photos were used on NPR's online platforms and digital stories and slide shows.

They were traveling near Marjah when the convoy was attacked on Sunday morning Afghan time. Baz Gul Mujahid, a local Afghan official, described the attack as an ambush, and said the journalists' vehicle had been struck by an 82mm rocket.

There was no immediate information available on any injuries or deaths among the Afghan army unit.

Ashraf Ghani, the president of Afghanistan, attributed the killings to "Taliban brutality" and said "those who committed the heinous crime will be remembered as forces of hatred." The Taliban has not yet claimed responsibility for the attack.

Gallery: David Gilkey's photos from the frontlines

Gilkey and Tamanna's deaths were not announced until Sunday afternoon U.S. time because NPR officials notified family members first.

NPR staff members responded with shock when the news broke.

NPR CEO Jarl Mohn said in a statement, "Our hearts go out to his family, his friends and his colleagues out in the field."

"This is an unimaginable loss," "All Things Considered" host Audie Cornish wrote on Twitter. "David Gilkey was one of our greatest journalists."

CNN Senior International Correspondent Ivan Watson, who previously worked with Gilkey at NPR, said he was a "generous and patient mentor when it came to combat reporting."

"He was a master storyteller of all types and genres. Who else could come to a non-profit radio organization like NPR and carve out a niche for award-winning photography?" Watson said.

"Telling untold stories is what we do as radio journalists. David Gilkey showed them to you in his photographs that could move you to tears," "Morning Edition" producer Emily Ochsenschlager wrote.

She also highlighted Tamanna's contribution: "As someone who's traveled for NPR, I can attest to the importance of a translator. They are your rock, your ear."

Sultan Faizy, a freelance journalist and close friend of Tamanna, described him as "brave and committed to his profession."

"The only thing he was worried about was his family," Faizy said.

According to the Committee to Protect Journalists, 31 journalists and media workers have been killed in Afghanistan since 9/11. Most of the cases were confirmed to be work-related.

The most recent case, in December 2014, involved a suicide bombing in Kabul. Zubair Hatami, a local cameraman, was among the dead.

"We've lost too many journalists to these wars. It's extremely dangerous work. Yet it's work that has to go on," Oreskes said. "And there's nobody who believed in that more than David."

In a statement on Sunday night, the Committee to Protect Journalists' Asia program coordinator Bob Dietz said, "Even though much of the world's attention has shifted away, let no one doubt that Afghanistan remains a dangerous place for journalists -- local and foreign -- working to cover that protracted conflict. We are deeply saddened by the deaths of Zabihullah Tamanna and David Gilkey. There are too many journalists who have given their lives to tell the Afghan story."

-- Masoud Popalzai contributed reporting.