South Korean prosecutors in the espionage trial of a North Korea defector said Thursday they have withdrawn evidence allegedly forged by Seoul's intelligence agency.

The evidence, which includes Chinese immigration records and two documents issued by Chinese authorities, was earlier announced by the Chinese Embassy here as fake. The finding prompted speculation that the National Intelligence Service (NIS) had fabricated the key evidence and handed it over to the prosecution to frame the defector.

The Seoul Central District Prosecutors' Office said that it has decided to omit the three documents from the ongoing appeals trial of the 34-year-old defector, Yoo Woo-seong, but will not drop charges against him.

"After reviewing relevant records, the prosecution decided that Yoo's spy charges will be acknowledged by the court without the (three) documents," said Yoon Woong-keol, a prosecutor close to the investigation.

"The prosecution office will try its best in proving Yoo's espionage charges," Yoon said, adding that the prosecution will hand in new evidence to the Seoul High Court.

The prosecution office will submit an audio file of the testimony of Yoo's younger sister, who had first raised spy charges against him in the trials, Yoon said.

The high-profile case involving Yoo began when prosecutors charged him with carrying out espionage for Pyongyang's spy agency.

Prosecutors alleged that Yoo collected detailed information on some 200 defectors in the South while he was working at the Seoul city government and relayed it to the North.

After a local district court acquitted Yoo of espionage charges in August 2013, allegations arose that the NIS forged evidence to help the prosecution appeal his innocence.

At the request of the Seoul High Court, the Chinese Embassy determined the documents to be fake, prompting the prosecution office to launch a separate investigation team to look into the allegations.

Already two people, including one NIS agent, have been put under arrest for their alleged role in forging the documents.

"We cannot yet confirm whether all of the three documents were forged as there is an ongoing investigation into the forgery," said Yoon.

The prosecution office also has not yet concluded whether the alleged forgery was part of a much bigger operation orchestrated by the leadership of the NIS, which is still reeling from allegations that it attempted to sway public opinion ahead of the 2012 presidential election.

According to sources at the prosecution office, investigators are expected to announce the results of their investigation into the alleged forgery sometime early next week.