As the vehicles roll slowly forward, the tooting of car horns rises to a crescendo in apparent celebration of victory in battle and the sound of whooping and gunshots can be heard.

A police officer in a blue uniform drives alongside, smiling as the Humvees are waved forward by a pedestrian in civilian clothes and head towards two large arches that span the road. The bodies are being paraded like prize stags after a hunt.

The film, which appears to have been made with a mobile phone, was passed to The Sunday Times by a senior official close to Moqtada al-Sadr, the radical Shi’ite cleric who leads the Mahdi Army militia.

The official said it had come from Basra and showed the bodies of two Mahdi fighters who died after the Iraqi army launched an offensive in the southern port city in March with the aim of liberating it from the grip of warring militias.

There was no way to corroborate the official’s information or to identify the dead men as Mahdi fighters, but the vehicles bear Iraqi army markings and the arches glimpsed in the film resemble a Basra landmark.

The release of the video follows the Iraqi army’s success in taking a Mahdi stronghold in Basra and coincides with intense fighting between the two sides in the Sadr City suburb of Baghdad, where more than 1,000 people have been killed in little more than a month.

Yesterday a truce was announced after Sadr’s officials agreed to let the Iraqi army enter the suburb and confiscate heavy weapons in return for security guarantees. However, Mahdi sources said the parading of corpses would increase distrust of Nouri al-Maliki, the prime minister, and his army, which is largely trained and supported by the United States.

“The Mahdi will not surrender its weapons to such an army,” said one commander. “They say we are outlaws but this video just goes to prove that Maliki’s forces are nothing more than a militia. They will never take Sadr City unless they wipe out each and every one of us.”

A second video obtained from the same source purports to show prisoners being beaten in a police station in the Shi’ite holy city of Karbala, south of Baghdad.

An Iraqi lawyer who has advised Maliki’s government said the two videos showed soldiers and police in serious breach of the law. “Desecratinga corpse is prohibited in law, even if he had been the worst criminal on earth,” said Maen Zaki, a former member of a government committee that handled legal issues arising from operations to restore order in Baghdad.

Major-General Mohammed al-Askari, an Iraqi defence ministry spokesman, said: “All videos and films can be manipulated these days. However, we do not tolerate anyone who defiles a body or abuses a person’s human rights. We will exercise the maximum punishment.”