SEOUL (AFP) - South Korea and the United States said Tuesday they would kick off their annual joint military exercises on March 2, setting the stage for a likely surge in tensions with North Korea.

Pyongyang had offered a moratorium on nuclear testing if this year's joint drills were cancelled - a proposal rejected by Washington as an "implicit threat" to carry out a fourth nuclear test.

The Key Resolve and Foal Eagle exercises are a perennial source of volatile tensions on the divided Korean peninsula.

Seoul and Washington insist they are defensive in nature, but they are regularly condemned by Pyongyang as provocative rehearsals for invasion.

The Key Resolve exercise lasts just over a week and is a largely computer-simulated exercise, while the eight-week Foal Eagle drill involves air, ground and naval field training.

Both exercises will begin on March 2, with Key Resolve lasting until March 13 and Foal Eagle winding up on April 24, a South Korean Defence Ministry spokesman said.

North Korea has regularly resorted to missile tests and high-decibel bellicose rhetoric in expressing its displeasure with the exercises in the past.

In 2013, the drills fuelled an unusually sharp and protracted surge in military tensions, with Pyongyang threatening a pre-emptive nuclear strike, and nuclear capable US stealth bombers making dummy runs over the Korean peninsula.

In a speech to the ruling party's Central Military Commission at the weekend, North Korean leader Kim Jong Un directed the army to ensure its combat readiness in order to react to "any form of war ignited by the enemy."