Six South Koreans recently repatriated from North Korea had sneaked into the North in search of better lives but ended up detained for up to 45 months for illegal entry, Seoul officials said Monday.

South Koreans defecting to impoverished, totalitarian North Korea are rare. In contrast, more than 25,000 North Koreans have fled to the South for political and economic reasons since the end of the 1950-53 Korean War. The two Koreas bar ordinary citizens from freely traveling across their mutual border, and in September South Korean soldiers fatally shot one of their citizens they thought was trying to enter North Korea.

North Korea sent back the six men and a woman's corpse on Friday in an unusual action seen as an attempt to improve strained ties between the rival countries after the North's abrupt cancellations last month of reunions of families separated by the war. Tensions spiked dramatically this past spring over North Korea's repeated threats of nuclear war.

A South Korean security official said Monday that the men told investigators they entered North Korea between 2009 and 2012 by walking over frozen rivers from China, or swimming after jumping off a Chinese cruise ship on a border river.

The men had vague hopes that they could have better lives in North Korea after suffering business failures and family troubles, or engaging in pro-North Korea activities in South Korea, said the official, who requested anonymity because an investigation is still under way.

One person who posted pro-North Korea online messages believed he would be treated better in North Korea after finding that his pen name had been reported by the North's main Rodong Sinmun newspaper, the official said.

The six men were separately detained in North Korea for 14-45 months for questioning. One said he was constantly held in solitary confinement, while another said he didn't receive medical treatment for a kidney stone, according to the South Korean official.

South Korean authorities are seeking a warrant to formally arrest the six men, aged between 27 and 67, for a more thorough investigation, the official said. In South Korea, anyone who defects to North Korea can be punished by up to 10 years in prison under the country's anti-North security law.

The results of a preliminary investigation were provided to South Korea media, and another South Korean official confirmed the substance of the reports.

Seoul estimates that hundreds of South Koreans have been kidnapped and detained by North Korea since the Korean War ended with an armistice, not a peace treaty. (AP)