SEOUL: South Korean and US troops began large-scale military exercises on Monday in an annual test of their defences against North Korea, which called the drills "nuclear war moves" and threatened to respond with an all-out offensive.

South Korea said the exercises would be the largest ever following North Korea's fourth nuclear test in January and a long-range rocket launch last month that triggered a UN Security Council resolution and tough new sanctions.

Isolated North Korea has rejected criticism of its nuclear and rocket programmes, even from old ally China, and last week leader Kim Jong Un ordered his country to be ready to use nuclear weapons in the face of what he sees as growing threats from enemies.

The joint US and South Korean military command said it had notified North Korea of "the non-provocative nature" of the exercises involving about 17,000 American troops and more than 300,000 South Koreans.

South Korea's Defence Ministry said it had seen no sign of any unusual military activity by North Korea, but Seoul's spy agency said it would hold an emergency meeting on Tuesday to check readiness against cyber attack after detecting evidence of North Korean attempts to hack into South Korean mobile phones.

North Korea's National Defence Commission said the North Korean army and people would "realise the greatest desire of the Korean nation through a sacred war of justice for reunification", in response to any attack by US and South Korean forces.

"The army and people of the DPRK will launch an all-out offensive to decisively counter the US and its followers' hysterical nuclear war moves," the North Korean commission said in a statement carried by the North's KCNA news agency.

North Korea, or the Democratic People's Republic of Korea (DPRK) as it is officially known, routinely issues threats of military action in response to the annual exercises that it sees as preparation for war against it.

The US State Department said the United States took North Korean threats to use nuclear weapons seriously and urged Pyongyang to halt provocations.

"(We) again call on Pyongyang to cease with the provocative rhetoric, cease with the threats and quite frankly, more critically, cease with the provocative behaviour, the actual conduct, that has led to yet another round of international sanctions," US State Department spokesman John Kirby told a regular briefing.

Russia's Foreign Ministry said it was opposed to South Korean-US military exercises as a way of pressuring North Korea but said Pyongyang's reaction was also unacceptable.

"The development of the situation on the Korean peninsula and around it is causing a growing concern," the ministry said in a statement that called on the parties involved to show restraint.

Chinese Foreign Ministry spokesman Hong Lei said Beijing was "deeply concerned" about the exercises and added: "We urge all sides to keep calm, exercise restraint and not escalate tensions."

The latest UN sanctions were drafted by the United States and China as punishment for North Korea's recent tests.

China though has been alarmed by discussions between South Korea and the United States on possible deployment of a new US anti-missile system to South Korea.

The South Korean and US militaries began formal talks on Friday on the Terminal High Altitude Area Defence (THAAD) system, which China sees as a threat to its strategic deterrence.