North Korean hackers have stolen hundreds of classified military documents from South Korea, including detailed wartime operational plans involving its US ally, according to a local media report.

Rhee Cheol-hee, a lawmaker for the ruling Democratic party, said the hackers broke into the South’s military network in September last year and gained access to 235 gigabytes of sensitive data, the Chosun Ilbo daily reported.

Among the leaked documents was Operational Plans 5015, for use in case of war with the North and including procedures for “decapitation” attacks on leader Kim Jong-un, the paper quoted Rhee as saying.

Rhee, a member of parliament’s defence committee, could not be reached for comment, but his office said he had been quoted correctly.

The report comes amid heightened fears of conflict on the Korean peninsula, fuelled by Donald Trump’s continued threats of military action against Pyongyang to tame its weapons ambitions.

In his latest tweet over the weekend, Trump reiterated that diplomatic efforts with North Korea had consistently failed, adding: “Only one thing will work.”

A Pentagon spokesman, Col Rob Manning, said he was aware of the report, but declined to confirm or deny any aspect of it.

“I can assure you that we are confident in the security of our operations plans and our ability to deal with any threat from North Korea,” Manning told Pentagon reporters.

“I am not going to address whether or not that [hack] has occurred. What I am going to tell you is that the [South Korea]-US alliance, that bilateral entity, is there to deal with those types of situation and safeguard against them.”

Citing Seoul’s defence ministry, Rhee said that 80% of the leaked documents had yet to be identified.

But the contingency plan for the South’s special forces was stolen, he said, as well as details about annual joint military drills with the US and information on key military facilities and power plants.

A ministry spokesman declined to confirm the report, citing intelligence matters.

In May, the ministry said North Korea had hacked into Seoul’s military intranet but did not say what had been leaked.

Pyongyang has a 6,800-strong unit of trained cyberwarfare specialists, according to the South Korean government. It has been accused of launching high-profile cyber-attacks, including the 2014 hacking of Sony Pictures.

The Chosun Ilbo story was the second report on Tuesday of military-related cyber-attacks in the Asia-Pacific.

Australia’s government said separately an unidentified defence contractor had been hacked and a “significant amount of data” stolen.

There were 47,000 cyberincidents in the last 12 months, a 15% increase from the previous year, the minister for cybersecurity, Dan Tehan, said in Canberra as he unveiled a report by the Cyber Security Centre.

The defence contractor was exploited via an internet-facing server, with the cybercriminals using remote administrative access to remain in its network, the report said.

The hacker was reportedly based in China, but Tehan told the Australian Broadcasting Corporation: “We don’t know and we cannot confirm exactly who the actor was.”