North Korea's hacking groups have broken into at least two South Korean manufacturers of chipmaking equipment, as the country looks to evade sanctions and turn out its own semiconductors for weapons programmes, South Korea's spy agency said on Monday.

The news comes after South Korean President Yoon Suk Yeol warned North Korea could stage provocations, such as cyber attacks or spreading fake news, to interfere in April's parliamentary elections.

The National Intelligence Service (NIS) said the South Korean firms had been a key target of North Korean hackers since late last year, and called for tougher security.

North Korea penetrated the servers of two companies in December and February, stealing product design drawings and photographs of their facilities, the NIS said.

"We believe that North Korea might possibly be preparing to produce its own semiconductors in the face of difficulties in procuring them due to sanctions," it said in a statement.

Also driving the North's efforts could be higher demand from its satellite, missile and other weapons programmes, it added.

Pyongyang has always denied involvement in cybercrimes, although North Korea has been blamed for cyberattacks netting millions of dollars.

The hackers employed a technique called "living off the land," which minimises malicious codes and uses existing, legitimate tools installed within the servers, making it difficult to detect with security software, the NIS said.

The US and South Korea begin large military drills to boost readiness against North's threats

South Korea and the United States began large annual military exercises Monday to bolster their readiness against North Korean nuclear threats after the North raised animosities with an extension of missile tests and belligerent rhetoric earlier this year.

The South Korean and U.S. forces began a computer-simulated command post training called the Freedom Shield exercise and a variety of field exercises for an 11-day run, the South Korean Defense Ministry said.

North Korea had no immediate response to the major annual drills it regards as a rehearsal for invasion. The North has staged provocative weapons tests in the past in reaction to its adversaries' joint drills.

South Korea’s military said last week that it would conduct 48 field exercises with the U.S. forces this spring, twice the number conducted last year, and that they would involve live-firing, bombing, air assault and missile interception drills.

Since early 2022, North Korea has conducted more than 100 rounds of missile tests to modernize its arsenal as talks with the United States and South Korea have been stalled for an extended period. In response, the United States and South Korea have expanded their training exercises and increased the deployment of powerful U.S military assets such as aircraft carriers and long-range nuclear-capable bombers.

This year, North Korea carried out six rounds of missile tests and barrage of artillery firing drills. Its leader Kim Jong Un also said North Korea would scrap its long-standing goal of peaceful unification with South Korea and take a more aggressive military posture along the disputed sea boundary with South Korea. He also vowed to “annihilate” South Korea and the United States if provoked, a threat that he had previously issued.

The North Korean steps raised worries that it might make provocations along the tense Korean sea and land borders. But experts say the prospect for a full-blown attack by North Korea is dim as the North knows its military is outmatched by U.S. and South Korean forces.

North Korea’s moves to raise tensions are likely related to upcoming elections planned by its rivals: the U.S. presidential election in November and South Korea's parliament election in April. North Korea believes an advanced nuclear arsenal will increase its leverage in future diplomacy and it can win concessions like the easing of international sanctions, experts say.