Image copyright Reuters Image caption All healthy adult South Korean males must serve about two years in the army

Two South Korean special forces soldiers have died during training, officials say, amid growing concern over abuse in the country's military.

The two men, both in their early 20s, collapsed during captivity training on Tuesday, Yonhap news agency said.

They died of apparent suffocation, with a third man needing treatment, it said.

The news comes a day after charges against four soldiers over the death of a conscript were upgraded from manslaughter to murder.

The young soldier - who prosecutors say had been bullied - died in April after being struck on the chest, causing him to choke on food.

Additionally, at least four young conscripts have killed themselves since July, while in June a soldier shot himself, but survived, after killing five members of his unit.

The incidents have sparked a wave of concern and anger over a culture of violence within the military, in which all South Korean men must serve for about two years.

Militarised nation

This latest incident happened at a Special Warfare Command unit in Jeungpyeong, south of Seoul.

The men were being trained on what to do if captured, Yonhap said. Citing officials, it said they had spent more than an hour on their knees with their hands tied behind their backs wearing hoods over their faces.

Two of them "had breathing problems during the mock captivity training when their heads were wrapped in cloth, and died after being sent to hospital", said a Special Warfare Command spokesman.

Yonhap quoted another official as saying the training programme had been adopted from the US, Britain and Australia and "there must have been some sloppiness in carrying out the training".

Last month South Korean President Park Geun-hye called for measures to ensure violence within the military was brought under control.

The nation remains technically at war with North Korea and the demilitarised zone (DMZ) which divides the two nations is one of the most heavily guarded areas in the world.

South Korea's conscription period is one of the longer globally, with most conscripts aged in their early 20s.