By Yimou Lee

TAIPEI (Reuters) -Taiwan is investigating a possible leak of official documents including diplomatic cables and classified reports on the island's sensitive bid to join a global trade pact, according to two officials familiar with the probe.

One official said initial findings showed some parts of the documents, posted on online message board 8kun and reviewed by Reuters, are real while bits were forged, without giving details.

A second official said parts of the documents appeared to be "authentic" and they could not immediately determine the origin of the documents shared on the internet.

The officials asked not to be identified due to the sensitivity of the matter.

In response to Reuters' questions, Taiwan's National Security Bureau (NSB) said it was aware of "suspected government documents" posted online and that relevant government authorities were "investigating and dealing with the source of such information".

The NSB said it was also investigating whether China, which claims Taiwan as its own, was involved in the incident.

China's Taiwan Affairs Office did not immediately respond to a request for comment.

In a statement, Taiwan's foreign ministry largely echoed the NSB comments adding that it "was difficult to distinguish between true and false information online" and urged vigilance about sharing information from unknown sources.

The incident comes at a delicate time for the island, which is due to hold elections early next year and is seeking to join the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) trade pact despite objections from China which is also seeking to join.

The documents posted online include what purports to be a classified "security assessment" in October by Taiwan's top intelligence agency, the National Security Bureau, on the island's CPTPP bid.

They include alleged diplomatic cables from Taiwan's de facto embassies in Japan and Vietnam about the CPTPP applications by China and Taiwan, and another classified report this year by Taiwan's de facto embassy in Washington on its trade negotiations with the United States.

Reuters has seen the documents but could not independently verify their authenticity.

Taiwan and China both applied in 2021 to join the CPTPP, a trade pact between 11 countries - Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam. Britain is also set to join.

Beijing has said it strongly opposes Taiwan's membership because Taiwan is part of China and therefore ineligible to join international bodies on its own. Taiwan is, however, a member of the World Trade Organization, designated as a separate customs territory called Taiwan, Penghu, Kinmen and Matsu.

Taiwan strongly rejects Chinese sovereignty claims and says only Taiwanese people can decide their future.

The island of 23 million people is growing increasingly concerned about hacks and cyber attacks as it gears up for the presidential election, due in January.

President Tsai Ing-wen, whose ruling party is seeking to head off a challenge from opposition parties pledging friendlier ties with Beijing, has repeatedly warned of China's attempts to influence public opinion ahead of the vote.

(Reporting By Yimou Lee; Editing by John Geddie, Lincoln Feast, William Maclean)