"Hands behind your backs! No talking!" a soldier shouted, while a German shepherd dog sniffed through their belongings laid out behind them.

It is here, just before they are processed, that the risk of violence or someone escaping is at its highest, Lieutenant Colonel Hakan Isacsson explained to AFP during a press visit.

Rattled by Russia's invasion of Ukraine, the Swedish armed forces were relearning how to handle prisoners of war (POWs), just in case.

Part of Aurora 26, the largest military exercise organised in Sweden, involving 18,000 soldiers from 13 countries, the 11-day segment devoted to the handling of POWs was rare.

It attracted numerous foreign visitors, mainly armed forces personnel from Canada, the Czech Republic, the United States and Ukraine.

The growing number of conflicts around the world, notably the war in Ukraine, as well as Sweden's new role as a NATO member prompted the Scandinavian country to hold the exercise, Isacsson said.

- Actors as POWs -

Around 300 extras -- including actors, soldiers and civilians -- were brought in to play the prisoners.

Some took to the part with great seriousness, with cold stares and eyes fixed on the ground. Others exchanged mischievous glances and struggled not to burst out laughing.

In total, the POW exercise included 500 people. It was the first such drill held in Sweden in 30 years.

"The scenario here is that we are in a rear position. War has just broken out on NATO's eastern border, in Finland. The prisoners of war here are being transported from the front through Sweden," said Major Johannes Kirchheim, who oversaw the exercise.

Located a little over 20 kilometres (12 miles) north of the city of Orebro, the Halahult sanatorium, which was used in the early 20th century to treat tuberculosis patients, can house up to 500 people.

- Target -

Inside, the prisoners were split into two groups: women on one side, men on the other.

After registration, the detainees, each identified according to their military rank or civilian status, were then taken to their rooms. Officers and low-ranking soldiers get housed seperately -- a requirement under the Geneva Convention.

"It may well be that the soldiers hate their officers. Or that the officers hate the soldiers, depending on what has happened. That's why we have to separate them. It's also about preventing them from regrouping or starting to hatch plans," Isacsson explained.

An observer from the Swedish Red Cross accompanied the soldiers to ensure that international legal obligations were respected, in particular that prisoners were treated humanely and with dignity.

For soldier Iza Andersson, it was crucial to keep in mind that detainees are often "very shaken and extremely vulnerable".

"It is essential to look after them appropriately, because at that point they are at their psychological lowest," she said.

"You have to know how to be firm when firmness is necessary, and more understanding in other situations, because it is hard to imagine what they have actually been through and what they have seen," she told AFP.

Ukrainian soldiers invited to the exercise also shared their experiences, stressing that the detention centre could itself be a military target.

That was an eye-opener for Isacsson. The Ukrainians, he said, quickly told them: "'Don't expect this to be a peaceful site.'"