SEOUL (Reuters) - South Korea's Supreme Court upheld a sedition conviction and nine-year jail term against a former leftist member of Parliament for inciting armed revolt, backing the government's crackdown on pro-North Korea activism under a controversial state security law.

Conservative President Park Geun Hye this month rejected the idea that the National Security Law should be abolished because North Korea remained a threat, affirming the government's push to prosecute those who vocally support the reclusive North's regime.

The Supreme Court on Thursday upheld an appeals court ruling against Lee Seok Ki of the now-defunct United Progressive Party for sedition as well as violating the National Security Law by organising secret party meetings and discussing a plot to stage rebellion.

His party has been expelled from Parliament and was ordered to disband in a separate ruling by the Constitutional Court in December for trying to overthrow free democracy and establish a pro-communist government. Lee has denied the charges, saying his comments were taken out of context and misinterpreted. His initial sentence of 12 years was cut to nine years by an appeals court.

Members of the party had been seen refusing to sing the South Korean anthem at public events. The party in the past won roughly 1 per cent of the popular vote and was seen as supporting the North's political aims.

On Wednesday, the government said it would seek out and crack down on "those who deny our constitutional values" and step up monitoring of pro-North Korea activities online.

South Korean media said the government was reviewing a revision to strengthen the security law.

Last week, prosecutors arrested a unification activist for making favourable comments about the North in speeches.

A South Korea-born American was deported on Jan 10 after speaking positively of life in North Korea in speeches around the country, as well as in online posts.

The National Security Law, enacted after the two Koreas were split at the end of World War Two but before the 1950-1953 Korean War, prohibits South Koreans from publicly praising the North Korean regime.

It is considered obsolete by liberal critics, who say it is often used by conservative governments to stifle political opposition and suppress freedom of speech.