North Korean hackers have broken into South Korean chip equipment makers, according to South Korea's spy agency.

Pyongyang is trying to make semiconductors for its weapons programmes, the National Intelligence Service (NIS) says.

It comes a month after President Yoon Suk Yeol warned North Korea may stage provocations such as cyber attacks to interfere with upcoming elections.

Last year, North Korea hacked into the emails of an aide to President Yoon.

"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," the NIS said in a statement.

It added that Pyongyang's efforts could be driven by the need to have chips for its weapons programmes, including satellites and missiles.

The NIS believes North Korea penetrated the servers of two chip equipment companies in December and February, stealing product designs and photographs of their facilities.

It also warned other companies in the chip making industry to take precautions against cyber attacks.

However, the spy agency did not name the firms affected and or suggest that North Korea was able to obtain anything of value.

The NIS said South Korea's companies had been a key target of North Korean hackers since late last year.

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

Last month, President Yoon's office said that the breach of an aide's email account was caused by a violation of security regulations and that its official system had not been hacked.

Pyongyang has always denied involvement in cyber-crimes but Seoul has blamed North Korean hackers for stealing large sums of money, often in cryptocurrency, to fund the regime and its nuclear weapons programme.

North Korea is estimated to have stolen as much as $3bn (£2.36bn) since 2016.

It is also thought to carry out hacks with the purpose of stealing state secrets, including details of advanced weapons technology.

The country, which is subject to extreme international sanctions, is becoming increasingly more sophisticated in the way it carries out cyber attacks.