SEOUL, South Korea — North Korea demanded Sunday that South Korea release an activist whose arrest the North says demonstrates Seoul's confrontational, anti-unification policy.

The Christian activist. the Rev. Han Sang-ryol, was arrested Friday upon his return home from a more than two-month visit to North Korea, where he accused South Korean President Lee Myung-bak's government of raising tension with the North by discarding past rapprochement accords.

"The Lee group should set free at once Rev. Han who had been thrown into behind bars," a North Korean government committee said in a statement carried by the official Korean Central News Agency.

The arrest showed that Lee is "an anti-unification, confrontational fanatic," it said.

In Seoul, police and intelligence officers continued investigating Han on Sunday to determine if his activities violated the National Security Law, according to police.

The security law prohibits South Koreans from having unauthorized contact with North Korea and supporting or praising the North's communist regime, while a separate law on inter-Korean exchange also bars South Koreans from visiting the North without government permission.

Calls to Han's Seoul-based Korea Alliance of Progressive Movements were unanswered Sunday.

Police say Han could face up to seven years in prison if convicted of violating the security law and up to three years for violating the inter-Korean exchange law.

Han's trip came amid heightened tension over the sinking of a South Korean warship in March which Seoul blames on North Korea. The North denies responsibility. Han said in Pyongyang that Lee's government, not North Korea, must be blamed for the sinking in which 46 sailors died.

The two Koreas officially remain at war because the 1950-53 Korean War ended with an armistice, not a peace treaty. Their relations flourished following the first-ever summit in 2000 between their leaders, who agreed to work toward peace and reconciliation. But ties have soured again since Lee took office in early 2008 with a tougher policy toward the North.