Prosecutors launched a probe into allegations that evidence against a former civil servant accused of spying for North Korea was forged.

"We will carry out the investigation in a thorough and swift manner," a senior prosecutor said.

The announcement came after defense lawyers claimed a key piece of evidence against Yoo Woo-seong -- a Chinese national in North Korea who arrived in South Korea by disguising himself as a North Korean defector -- was fabricated.

The 34-year-old suspect was indicted last year on charges of handing over personal information of more than 200 North Korean defectors to North Korea's spy agency.

A local court acquitted Yoo of espionage charges in August, citing the lack of credibility of testimony by a key witness in the case.

Prosecutors appealed the case and submitted Chinese immigration records showing that Yoo entered North Korea on May 27, 2006 before crossing back to China on June 10, 2006.

Yoo had claimed that he visited North Korea for his mother's funeral but never went back to the North after May 27, 2006.

At the request of the Seoul appeals court, the Chinese Embassy in Seoul examined the documents and announced last week that they are fake, according to defense lawyers.

The immigration records were a key piece of evidence in the case since prosecutors claimed Yoo was recruited by the North's spy agency during his stay in the North and had since engaged in espionage activity. (Yonhap)