White House chief of staff John Kelly ─ a former general whose son died in Afghanistan ─ made an emotive defence of Donald Trump on Thursday, amid a scandal over the president's phone call with a military widow.

Kelly, who spent nearly four decades in the Marine Corps, said he had advised Trump not to call the families of four servicemen killed in Niger personally but was disgusted by how the issue had become politicized.

He blamed the Democratic lawmaker who made public the contents of a call between Trump and widow Myeshia Johnson.

“I was stunned when I came to work yesterday morning and brokenhearted at what I saw a member of Congress doing,” he said.

“A member of Congress who listened in on a phone call from the president of the United States to a young wife,” he said. “Absolutely stuns me. And I thought at least that was sacred.”

“The only thing I could do to collect my thoughts was to go and walk among the finest men and women on this Earth. And you can always find them. Because they're in Arlington National Cemetery.”

Kelly said he “went over there for an hour and a half, walked among the stones, some of whom I put there because they were doing what I told them to do when they were killed.”

Trump kicked off the furore early this week by falsely claiming that Barack Obama and other former US leaders did not call the families of fallen soldiers.

He returned to the subject in an interview with Fox News radio and brought up his chief of staff Kelly, whose son, a Marine Corps lieutenant, was killed by a landmine in Afghanistan in 2010.

“You could ask General Kelly 'Did he get a call from Obama?'” Trump said.

'It still hurts'

It was then alleged that during a call Trump had offended the pregnant widow of Sergeant La David Johnson, 25, who was one of four US servicemembers killed in a jihadist ambush October 4.

US Defense Secretary Jim Mattis said on Thursday that Johnson's body was “found later” by non-American forces following the ambush, which is thought to have been conducted by jihadists in an area where an affiliate of the militant Islamic State (IS) group operates.

As is routine in incidents where troops are killed, the Pentagon has opened an inquiry into the soldiers' deaths.

The details of Trump's call with Johnson's widow were released by Frederica Wilson, a Democratic congresswoman from Florida.

“I didn't hear the whole phone call, but I did hear him say, 'I'm sure he knew what he was signing up for, but it still hurts,'” she recalled, sparking another round of controversy.

The soldier's mother also suggested that the president struggled to convey an empathetic tone.

“President Trump did disrespect my son and my daughter and also me and my husband,” Sergeant Johnson's mother, Cowanda Jones-Johnson, told the Washington Post.

Kelly said he had advised Trump not to make the calls: “My first recommendation was he not do it. Because it's not the phone call that parents, family members are looking forward to.”

But, after taking advice, Kelly said Trump “called four people the other day and expressed his condolences in the best way he could.”

“And in his way tried to express that opinion that he's a brave man, a fallen hero, he knew what he was getting himself into because he enlisted... and was where he wanted to be, exactly where he wanted to be with exactly the people he wanted to be with when his life was taken. That was the message.”

Kelly also noted that a president does not always call ─ particularly during high-casualty wars ─ but they do write letters to the family.

“Typically the only phone calls the family receives are the most important phone calls they could imagine, and that is from their buddies."