South Korea's top court has acquitted a man accused of sympathising with North Korea by retweeting posts by its government, arguing that his action poses little threat to national security.

Park Jeong-Geun, a 26-year-old photographer, was arrested and charged in 2012 of violating the National Security Law by retweeting posts by the North's official Twitter account.

Under the notorious anti-communist law, South Koreans are banned from activities deemed to be praising or sympathising with the North, which is still technically at war with the South.

A district court handed Park a suspended jail term but an appellate court later found him innocent.

The supreme court of Korea upheld the acquittal, after prosecutors appealed the ruling, saying his actions "did not pose tangible threats to national security."

Park has argued his retweets, including posts like "Long Live General Kim Jong-Il," were meant to ridicule the North's leaders and its rigid Stalinist system rather than praise them.

The isolated North joined Twitter in 2010 and has posted 11,000 tweets that mostly blast its major foes – the South and the US – and praise its ruling Kim family. It has nearly 20,000 followers.

Several rights groups voiced concerns over Park's case, with Amnesty International in 2012 saying the law had a "chilling effect" on freedom of expression.

The two Koreas technically remain at war after the 1950-53 Korean War ended with a ceasefire instead of a full peace treaty.

Cross-border ties have been particularly icy since 2008 when the conservative administration in the South replaced the previous governments known for their conciliatory posture towards Pyongyang.