Updated

SEOUL, South Korea -- South Korean prosecutors have indicted a Socialist Party member on charges that he reposted messages from a North Korean government Twitter account, a party official said Thursday.

Park Jeong-geun, a member of the small Seoul-based opposition party, was indicted earlier this week under the National Security Law, which makes it a crime to praise, sympathize or cooperate with North Korea, fellow party member Kim Seung-il said.

Amnesty International called the government's action "ludicrous." Calls to prosecutors seeking comment were unanswered late Thursday.

Park, 23, who runs a photo studio in Seoul, was detained last month for re-tweeting messages posted on the Twitter account of North Korea's Committee for the Peaceful Reunification of Korea, Kim said.

Among the messages Park reposted were "Long live Kim Jong Il" and "Dear General Kim Jong Il is the genius of the military and the symbol of victory who the entire world looks up to and follows," according to South Korea's Yonhap news agency.

Park, however, only re-tweeted the messages to promote freedom of expression and lampoon the North's government, Kim said. Park has been critical of the North's government and didn't intend to praise it, Kim said.

Amnesty International issued a statement Wednesday criticizing Park's indictment.

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

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

Indictments under South Korea's anti-North Korean security law have shot up since a conservative government took power in Seoul in 2008. More than 150 people were questioned and 60 charged in 2010, up from 39 questioned and 36 charged in 2007, according to official data.

In another sign of stepped-up enforcement, a South Korean government agency in December launched a team tasked with examining Facebook and Twitter posts and smartphone applications to cope with what it says is a growing volume of illicit content, including violations of the security law.

The two Koreas are still technically at war because the 1950-53 Korean War ended with an armistice, not a peace treaty. Ties plunged to one of their lowest points in 2010 after a South Korean warship exploded in disputed waters, killing 46. South Korea said the North torpedoed the warship; the North denied the allegation.

North Korea that year also fired artillery shells at a front-line South Korean island, killing four.

The North says a South Korean live-fire drill triggered the bombardment.