South Korean intelligence agents under investigation over a high-profile spy case have repeatedly denied allegations that they had been ordered to forge Chinese government documents to frame a North Korean defector for espionage, prosecutors said Friday.

The case centers around allegation that the National Intelligence Service obtained or produced fake Chinese immigration records and other key documents issued by Chinese authorities, and handed them over to the prosecution.

As part of the investigation, the Seoul Central District Prosecutors' Office in Seoul probing the case summoned several NIS agents to question whether they instructed a Chinese national, only identified by his surname Kim, to obtain the documents that were later announced by the Chinese Embassy here as fake.

Despite fresh allegations that a midranking NIS secret agent, who is also under arrest, gave a direct instruction to Kim to fabricate the documents, the agent testified that he was not aware of the forgery, prosecution officials said.

The prosecution office said it is mulling cross-examining NIS agents and Kim as they continue to give conflicting testimonies.

The case involving Yoo Woo-seong, a 34-year-old defector in South Korea of Chinese ethnicity, 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 indict him during appeals trials.

Investigators said that they are now focusing on further investigating whether the alleged forgery was part of a much bigger operation orchestrated by the leadership of the NIS.

According to other sources at the prosecution office, a high-ranking NIS official, surnamed Lee, is expected to appear at the prosecution office in southern Seoul sometime soon to face questioning.

Lee, who leads the anti-communist investigation unit within the NIS, was in charge of Yoo's case and oversaw all the documents that were later used in Yoo's appeals trial, the prosecution officials said. (Yonhap)