SEOUL (Reuters) – South Korea said on Wednesday it suspected North Korea of attempting cyber attacks against targets in the South, following a nuclear test by the North this month that defied United Nations sanctions.

South Korea has been on heightened military and cyber alert since the Jan. 6 test, which Pyongyang called a successful hydrogen bomb test, although U.S. officials and experts doubt that it managed such a technological advance.

“At this point, we suspect it is an act by North Korea,” Jeong Joon-hee, a spokesman of the South’s Unification Ministry, told a news briefing, when asked about reports that the North might have attempted cyber attacks.

Authorities were investigating, Jeong said, but did not provide further details.

Last week, South Korean President Park Geun-hye said the scope of threats from North Korea was expanding to include cyber warfare and the use of drones to infiltrate the South.

North Korea has been using balloons to drop propaganda leaflets in the South, amid heightened tension on the Korean peninsula since the nuclear test.

Since the test, there have been unconfirmed news reports that the computer systems of some South Korean government agencies and companies had been infected with malicious codes that might have been sent by the North.

Defectors from the North have previously said the country’s spy agency, run by the military, operates a sophisticated cyber-warfare unit that attempts to hack, and sabotage, enemy targets.

South Korea and the United States blamed North Korea for a 2014 cyber attack on Sony Pictures that crippled its systems and led to the leaks of unreleased films and employee data.

At the time, the company was set to release the film, “The Interview”, featuring a fictional plot to assassinate North Korean leader Kim Jong Un.

North Korea has denied the allegation.

In 2013, cybersecurity researchers said they believed North Korea was behind a series of attacks against computers at South Korean banks and broadcasting companies.

(Reporting by Ju-min Park and Jack Kim; Editing by Tony Munroe)