Pyongyang, North Korea - North Korea warned the US and South Korea that they will pay a "high price" for holding their annual spring military drills.

The Defense Ministry in Pyongyang described the 11-day Freedom Shield exercises, which began on Monday, as a threat that would be met with the appropriate response.

He did not say what counter-measures would be taken, however.

For its part, South Korea's Defense Ministry warned the North's isolated regime against undertaking "direct provocations."

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According to South Korea, preparation for nuclear threats from North Korea will be a core element of the readiness drills, which include computer simulations and training sessions in the field.

But North Korea on Tuesday accused the US armed forces of preparing an invasion together with South Korea.

Its People's Army will continue to observe the "adventurous actions of the enemy and undertake responsible military activities to control the unstable security environment on the Korean peninsula," it said.

South Korea and the US deny Pyongyang's accusations and insist their joint military exercises are exclusively for defensive purposes.

Tensions on the Korean peninsula have escalated considerably over the past two years. In response to the numerous tests of nuclear-capable missiles by North Korea, Seoul and Washington have further expanded their military cooperation.