A British soldier engaged in a gun battle in Iraq described on Wednesday how his colleagues punched and kicked a prisoner and stamped on the head of a dead Iraqi, and how he saw his sergeant empty a full magazine into the "twitching" bodies of fatally wounded insurgents.

Duncan Aston, a soldier in the 2nd Battalion of Princess of Wales Royal Regiment, gave a graphic description of what came to be known as the Battle of Danny Boy – at a British checkpoint in south-east Iraq in May 2004 – and described the actions of his platoon sergeant as "a bit sick and degrading".

He said the sergeant, Paul Kelly, attempted to fire his weapon into the bodies. "I am not too sure why – but that failed. And then he took my weapon and used my weapon."

Aston said Kelly fired around 30 rounds into a "pile" of Iraqi bodies from around 16ft-20ft away. "He put a full magazine of bullets into both bodies that had been twitching but he also fired into the bodies of the other dead gunmen in the ditch. The bodies of the two twitching gunmen stopped twitching."

Aston told the inquiry: "For all I knew, the Iraqis in the ditch may well have been dead already, but they were certainly going to die given the horrendous wounds to their chests where blood was squelching in and out of their chests. My thoughts therefore were that what he [Kelly] had done was a bit sick and degrading, but not something worse."

When questioned further about the incident, Aston added: "That is one of the memories I have most vividly from that day, Sgt Kelly doing that, because I don't know why he did it really." Asked if he had confronted Kelly about what he had done, he said: "No, I wouldn't dare."

In evidence to the al-Sweady inquiry into the incident – named after one of the alleged victims of abuse by British soldiers – he said two soldiers, Privates Steven Wells and Scott Barlow, punched and kicked an Iraqi on "different parts of his body, including his face, shins and ribs". However, he said it was not hard enough to cause serious injury.

He said he saw Wells enter a ditch where one of the dead Iraqis lay. "Pte Wells stamped on his head two or three times and screamed and shouted. I thought he was letting some anger go after the firefight." Aston added: "What I saw shocked me."

Aston, the first British soldier at the inquiry to accuse his colleagues of abuse, told the inquiry that he later saw the twitching bodies in the ditch. He added: "I had never seen anyone dead before and do not know whether in fact they were dead but I wouldn't describe them as alive either in the sense that to me they looked past the point of first aid and drifting somewhere between life and death."

He continued: "Having thought about it since, I believe they were probably close to death if not dead already. It was a very unpleasant sight."

Describing the intense gun battle – the first experience of being shot at for many of the young soldiers – Aston, who was 18 at the time, said at one point in his evidence to the inquiry: "I was crapping myself, to be fair."

Asked why he had not reported the behaviour he said he witnessed once he left the army in 2005, Aston replied:" I wanted to leave the army behind. I was ready to start another life so I didn't feel the need to report it."