A female North Korean spy arrested last week had used sex to secure sensitive information on Seoul's subway system, a South Korean prosecutor said today.

The 36-year-old identified only as Kim was arrested on Friday, eight months after she entered South Korea via China and Laos in the guise of a defector, Oh Se-In, a deputy chief prosecutor, told AFP.

She obtained information, including a list of emergency contacts for Seoul subway staff, from a 52-year-old former subway employee and handed it over to Pyongyang, he said.

"The information passed on is not a classified military secret. However, North Korea can use such information for terrorist attacks," Oh said.

"Passing key information on our subway system to North Korea is a violation of our national security law."

In China, the spy had run a cosmetics store and travel agency to secure personal data on South Korean tourists, he said.

The former subway employee, who met Kim online and visited China in 2006 to become her lover, was also arrested on charges of violating the security law, the prosecutor said.

"The spy had used sex to obtain information," he said, comparing Kim to Won Jeong-Hwa, a female North Korean spy who was arrested and jailed for five years in 2008.

Won, 36, had admitted to having sexual relations with a South Korean army officer to secure military secrets.

Almost all those fleeing hunger or repression in North Korea cross the border to China.

Some travel on to Southeast Asian nations in hope of eventual resettlement in South Korea, because Beijing has an agreement with Pyongyang to repatriate those refugees it catches as economic migrants.

Almost 18,000 North Koreans have arrived in South Korea since the 1950-1953 war.