Armed men attacked the home of the economy minister early on May 19 before moving on to the nearby Palais de la Nation that houses President Felix Tshisekedi's offices in the capital.

Around 40 people were arrested, with another four killed, out of a group made up of "several nationalities" army spokesman General Sylvain Ekenge said at the time.

The plot was led by Christian Malanga, a Congolese and "naturalised American" who was among those killed by security forces, Ekenge said.

But Human Rights Watch (HRW) said, "Congolese security forces killed Malanga in unclear circumstances hours after he seized the Palais de la Nation."

The New York-based group said it has located a video taken by Congolese soldiers that shows "presumed coup participants" being apparently shot at near the Palais.

"The government's response needs to be rights-respecting, which includes impartially investigating possible security force involvement in alleged summary executions," said Lewis Mudge, Central Africa director at HRW.

The video shows the banks of the Congo River near Tshisekedi's offices with seven men in civilian dress or without clothing, in and around a boat -- including one Caucasian man without clothing who is believed to be a foreigner who took part in the coup bid.

Six men in Congolese military fatigues are also seen in the video.

"A soldier shoots twice at one of the presumed coup participants, who then lies motionless," HRW said.

The recording also shows another man who jumps in the river "in an attempt to flee".

- 'Pool of blood' -

"The soldiers shoot at him several dozen times until a pool of blood appears in the water and he no longer comes up for air," the rights group said.

Other Congolese organisations including Lucha, a French acronym for fight for change, and Justicia Asbl also condemned the alleged extrajudicial killings.

"The government both has a responsibility to ensure the security of the country and to hold those responsible for the coup attempt accountable, based on international fair trial standards," added Lewis Mudge, Central Africa director at HRW.

The rights group said the coup attempt came after a prolonged period of the government having "significantly repressed" free expression, media freedom and peaceful assembly.

"Since 2020, the authorities have increasingly cracked down on journalists, human rights and democracy activists, critics of the government, and opposition party members and officials," HRW said.

Congolese journalist Stanis Bujakera was released from prison in March after spending six months behind bars.

He allegedly incriminated the country's military intelligence in the murder of opposition politician Cherubin Okende.

"The Congolese government needs to treat this crisis as an opportunity to demonstrate its commitment to human rights and the rule of law," Mudge said.