South Korea's spy agency on Tuesday accused North Korea of hacking into the smartphones of dozens of key officials, stealing text and voice messages and their call logs.

The announcement came a day after North Korea warned of nuclear strikes in response to the start of Seoul-Washington military exercises it views as an invasion rehearsal. This year's manoeuvres are the largest ever, meant to respond to the North's recent nuclear test and long-range rocket launch.

Tensions high as South Korea accuses Pyongyang of provocations

The National Intelligence Service said in a statement that the cyberattacks were made between late February and early March.

It didn't say which officials' phones were hacked or whether the stolen text and voice messages contained any sensitive information.

The statement said North Korea also unsuccessfully tried to hack into email accounts of South Korean railway officials earlier this year, in a preparation for cyberattack on the South's railway transport control system.

There was no immediate reaction from North Korea

Seoul has accused Pyongyang of launching a series of cyberattacks in recent years. Pyongyang has denied the claims.

Meanwhile, South Korea announced new unilateral sanctions against North Korea on Tuesday amid the escalation of tensions on the peninsula, including import-export restrictions

"First, we are expanding the scope of financial sanctions related to North Korea. Second, we are strengthening maritime regulations," said Lee Suk-joon, minister for government policy coordination.

Many alleged North Korean cyberattacks failed to infiltrate computer systems of businesses and government agencies.

But in several successful cases, also denied by North Korea, hackers destroyed hard-drive disks, paralysed banking systems, or disrupted access to websites. One of these attacks was so crippling that a South Korean bank was unable to resume online banking services for more than two weeks.

In 2014, the United States accused North Korea of a cyberattack on Sony Pictures over a movie depicting the fictional assassination of the North's leader, Kim Jong Un. North Korea has denied any involvement in the hacking.

Last year, South Korea said North Korea has a 6,000-member cyber army dedicated to disrupting the South's military and government. The figure was a sharp increase from a 2013 South Korean estimate of 3,000 such specialists.