Rights group Amnesty International Thursday urged the release of a South Korean activist accused of helping the "enemy" by re-tweeting messages from North Korea's official Twitter account.

Park Jeong-Geun, a Socialist Party activist, was arrested last month for re-tweeting messages such as "Long Live General Kim Jong-Il".

The 24-year-old says his re-tweets were meant to ridicule North Korea's leaders rather than support them.

He has been in custody since January 11 and could face up to seven years in jail under the strict National Security Law (NSL).

"This is not a national security case, it's a sad case of the South Korean authorities' complete failure to understand sarcasm," Sam Zarifi, Amnesty International's Asia-Pacific director, said in a statement.

"Imprisoning anyone for peaceful expression of their opinions violates international law but in this case, the charges against Park Jeong-Geun are simply ludicrous and should be dropped immediately," he said.

Amnesty said the party to which Park belongs has frequently criticised North Korea for exploiting its labour force and opposes its father-to-son succession.

Park has told journalists that his intention was to lampoon North Korea's leaders and its rigid Stalinist system.

Zarifi said the NSL has a "chilling effect" on freedom of expression.

"It is used not to address threats to national security, but instead to intimidate people and limit their rights to free speech. It should be reformed in line with human rights law, and if the government cannot do this, it must be abolished," he said.

Amnesty International said that despite the end of military rule in South Korea decades ago, "authorities have increasingly used the NSL to harass critics of the government's North Korea policies since 2008".