Won Jeong-hwa, 35, is suspected of collecting information, including photographs and the exact locations of key military installations and weapons systems, and handing it over to North Korean agents in China.

She was arrested last month, seven years after defecting from the North in 2001. Prosecutors believe she received professional training in espionage before being sent to the South.

They said several officers came into contact with the suspect but only one, a 25-year-old army captain, was found to have offered classified information.

Won's 63-year-old foster father has also been arrested on suspicion of helping her activities.

'We are working to expand our investigation to others as the case showed the North could have sent more spies posing as defectors,' Yonhap news agency quoted an unidentified official as saying.

Some 14,180 North Koreans have escaped and resettled in South Korea since the end of the 1950-53 war, with the number growing rapidly in recent years.

All of them undergo checks before being resettled.

Won's arrest came nearly two years after military intelligence began watching her, following a report by one of the officers she approached, intelligence officials told reporters.

More than 4,500 people have been exposed as spies for the North since the peninsula was divided in 1948, officials at the Defence Security Command said. -- AFP