South Korea warned Wednesday that it would retaliate against any provocation from North Korea, a day after the North threatened to tear up the armistice that ended the Korean War in 1953.

"If North Korea carries out provocations that threaten the lives and safety of South Koreans, our military will carry out strong and resolute retaliations," Army General Kim Yong-Hyun told reporters.

Kim's briefing followed North Korea's announcement on Tuesday that it would "completely declare invalid" the armistice agreement in response to moves to toughen UN sanctions after its recent nuclear test.

The announcement, attributed to the supreme command spokesman, also threatened an undefined "strike of justice" against a target of the North's choosing.

Because the armistice ending the 1950-53 conflict was not followed by a peace treaty, the two Koreas remain technically at war, with the ceasefire agreement theoretically the only barrier to a resumption of full hostilities.

The North has previously threatened to rip up the agreement and the truce has not prevented several bloody land and sea border clashes.

But the latest threat comes at a time of particularly heightened tensions on the Korean peninsula, following the North's successful launch of a long range rocket in December and its nuclear test last month.

Tuesday's army statement included a pointed mention of North Korea possessing "lighter and smaller nukes" than before.

The UN Security Council is expected to adopt tougher sanctions against the North this week -- a move likely to provoke a response from Pyongyang, which is also angry about joint US-South Korean military drills.

China called for restraint and said the armistice agreement played an important role in safeguarding peace on the peninsula.

A UN resolution was "still being discussed", its foreign ministry spokeswoman Hua Chunying said, adding Beijing supports the UN Security Council in making a "necessary and proportionate" response.

"At the same time we hope such action can be conducive to peace and stability and non-proliferation of the Korean peninsula," she said.

General Kim, the director general of operations of the Joint Chiefs of Staff, said South Korean retaliation would not only target the "origin of provocation" but also the North's commanding forces.

Last month the South's military released video footage of a newly deployed cruise missile that it could carry out high precision strikes on command centres anywhere in North Korea.

An annual US-South Korea exercise known as Foal Eagle began on March 1 and will run until April 30, involving more than 10,000 US troops along with a far greater number of South Korean personnel.

And a largely computer-simulated joint exercise called Key Resolve will be held from March 11-21.

The North's statement denounced the drills as the "most dangerous nuclear war manoeuvres... and the most undisguised military provocation".

"This land is neither the Balkans nor Iraq and Libya," the statement said, warning of the North's ability to launch "diversified precision nuclear strikes" in response.

South Korea's defence ministry says the North is expected next week to launch its own large-scale military exercise involving the three main branches of its armed forces.

The tension will be further ramped up if the UN Security Council goes ahead as expected this week and places North Korea under one of the toughest sanctions regimes ever ordered as a punishment for its nuclear test.

The sanctions, hammered out between the United States and the North's only major ally China, are believed to target the illicit activities of North Korean diplomats, banking relationships and bulk crash transfers.

They would also make searches of suspect ships compulsory and order UN members to refuse access to planes suspected of carrying banned material to or from North Korea.