An active-duty South Korean marine who made an unauthorised overseas trip -- reportedly an attempt to reach Ukraine -- has been arrested after arriving back home, Seoul's military said on Monday.

The serviceman, whose identity was withheld, left South Korea without permission while on duty on March 21, the Marine Corps said in a statement, adding that he was arrested immediately after returning home.

South Korea banned its nationals from travelling to Ukraine shortly before conflict broke out this year, citing safety concerns.

Active-duty servicemen are specifically banned from any overseas trips without prior approval and leaving without permission is considered desertion, which is a crime punishable by up to 10 years in prison.

"We will take stern measures in accordance with laws and regulations after investigating why he had left his duty," the Marine Corps said.

According to the Yonhap news agency, the man flew to Poland in an apparent attempt to join the Ukrainian army in its fight against Russia.

He was not, however, able to enter the war-torn country as access was denied at the Polish-Ukraine border, Yonhap reported.

The unidentified marine is widely believed to have uploaded a YouTube video on April 9 in which he said he had a "difficult time" in the South Korean armed forces.

"But I couldn't help but act when I heard about Ukraine, where people are faced with a much more difficult situation," he said in the footage, his face hidden from view.

'Volunteer fighter' killed

The defence ministry said the man was a conscript.

All able-bodied South Korean men are required to serve in the military for nearly two years, mainly due to the threat of conflict with nuclear-armed North Korea.

The two Koreas remain technically at war, as their 1950-53 conflict ended with an armistice and not a peace treaty.

Reports of bullying and other forms of abuse have long tainted South Korea's military. Such cases have previously resulted in suicides and deadly shooting sprees.

The announcement came days after Seoul's foreign ministry said it had received intelligence that at least one South Korean volunteer fighter -- who entered Ukraine without government permission -- has died.

Seoul banned its citizens from travelling to Ukraine in mid-February, but there are four South Korean nationals who are believed to be in the country as of Friday, the ministry added.

"We urge South Koreans who have entered Ukraine without permission to leave the country as soon as possible," the ministry said.

"There are serious concerns about your personal safety due to intensifying hostilities in southeastern Ukraine."