The wife of an Iraqi journalist shot dead at a Baghdad checkpoint for a "trivial reason" burst into tears when faced with his alleged killer in court on Monday.

Rafaa Jaafar demanded his killer receive a "fair punishment", speaking at the opening of the trial of an Iraqi Kurdish officer charged with murdering her husband Mohammed Bidaiwi.

Other witnesses recounted how an argument at the checkpoint had quickly escalated before the accused, Lieutenant Ahmed Braiyim, allegedly opened fire.

Bidaiwi, Baghdad bureau chief of Radio Free Iraq, was on his way to work on March 22 near the presidential complex on the edge of the heavily-fortified Green Zone when he was shot dead during a heated argument.

The trial marks a rare instance in Iraq where the murder of a journalist has been taken to court.

"I demand a fair punishment," 40-year-old Jaafar, herself a journalist, said in the courtroom.

"Criminals have orphaned his children ... for a trivial reason."

According to one of the witnesses who testified, Bidaiwi had been waiting in his car to enter the presidential complex, a sprawling compound that contains the homes of MPs and senior officials, as well as several offices, including those of Radio Free Iraq.

But a plainclothes officer in the Kurdish peshmerga force which guards the complex had attempted to cut in front of him in the queue at the checkpoint, sparking the argument.

"When the victim realised that the person who tried to bypass the queue was in the military, he apologised," said Wissam Jawaz Zair, the guard of an MP living nearby and who was on the scene.

"But the officer and another soldier beat the journalist."

At that point, other peshmerga members, including Braiyim, apparently made their way over.

"We tried to intervene to disengage them," Zair said, adding: "Lieutenant Ahmed suddenly desperately pulled out his gun in the middle of the soldiers ... and shot him (Bidaiwi) at close range."

Bidaiwi had been bureau chief for US-funded Radio Free Iraq since 2006 and was also an associate professor of journalism at Baghdad's Mustansiriyah University.

The station was established in 1998 and is a branch of US-funded Radio Free Europe/Radio Liberty, broadcasting in Arabic from Prague and Baghdad.

Iraq remains one of the world's deadliest countries for journalists and is routinely criticised for its poor record on media rights.

It frequently scores towards the bottom of press-freedom rankings, and tops the Committee to Protect Journalists' Impunity Index, which tracks unsolved murders of journalists.