An investigation has been launched following reports that four unarmed men were shot dead at close range by peacekeeping troops in Somalia.

Burundian troops serving under the African Union Mission in Somalia (Amisom) are alleged to have opened fire on four civilians on Tuesday after their convoy was targeted by a roadside bomb. Three of the men died at the scene, while the fourth died later in hospital.

Witnesses interviewed by Amnesty International reported that the troops drove over an improvised explosive device that detonated as they travelled through a district in the north of Mogadishu.

In response, the troops are alleged to have exited their vehicles and “arbitrarily shot” the four men, who were named as Ali Shire Ugas, Hassan Yusuf Siyad, Qasim Dahir Khayre and Ahmed Mohamud Basey. Three of the men were lorry drivers, while Basey was a tuk-tuk driver.

Amisom said in a statement that it has begun a “full-scale investigation” and that it will work closely with the federal government. The mission, which has about 20,000 troops in Somalia, has been fighting al-Shabaab for more than a decade. Amisom troops are due to gradually withdraw from Somalia, despite fears over increasing attacks by al-Shabaab.

Sarah Jackson, Amnesty International’s deputy director for east Africa, the Horn and the Great Lakes, said the reports were “deeply disturbing”, adding that any inquiry must be thorough and impartial. “It is also vital that this investigation fully protects and safeguards witnesses willing to come forward to testify,” said Jackson, who added that those responsible should be held to account.

In response to the deaths, dozens of demonstrators took to the streets of the capital, burning tyres and calling for the killers to be brought to justice.

One protestor, Abdulahi Mohamed, told Agence France-Presse: “We were protesting in solidarity with the victims and we need justice for these innocent victims. If nothing is done, we will take justice with our own hands.”

Amisom said in a statement that it would investigate the deaths, and that the results of its inquiry would be made public.

The statement added: “Prior to deployment in the theatre of operations, Amisom forces are always trained and mentored to strictly observe the provisions of international humanitarian law, as well as international human rights law and abide by the rules of engagement governing the conduct of military operations; in particular, the core principles of sacred respect for, and protection of, civilians.”