A Seoul city official, who was believed to have defected from North Korea and was arrested earlier this month for espionage, has been found to be of Chinese nationality, intelligence sources said Tuesday.

The 33-year-old suspect, identified only by his family name Yoo, arrived in South Korea in 2004, disguising himself as a North Korean defector. In 2011, he was hired by the Seoul city government as a two-year contract official.

The central government's main spy agency, the National Intelligence Service (NIS), put the man under formal arrest pending trial on Jan. 10 on charges that he had passed to North Korea sensitive information about thousands of North Korean defectors living in Seoul.

His job in the Seoul city government was helping North Korean defectors. This is the first time a defector-turned-public official has been arrested on espionage charges.

Intelligence sources said that Yoo used to live as a Chinese national in the North's Hamkyung province near the border with China and moved to South Korea in disguise as a North Korean defector, the sources said.

After relocating to South Korea, Yoo reportedly entered North Korea several times with his Chinese passport without reporting to South Korean authorities, they said.

The NIS is investigating if Yoo had any accomplices who had helped him pass on data to Pyongyang's spy agency, the sources added.

The case opens the government to public criticism for its lax security checks. More than 24,000 North Koreans are currently living in South Korea after fleeing their communist homeland. (Yonhap)