North Korea warned the United States and South Korea that they will pay a "high price" for holding their annual spring military drills.
The Defence 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 Defence Ministry warned the North's isolated regime against undertaking "direct provocations."
According to South Korea, defence against nuclear threats from North Korea will be a core element of the readiness drills, which include computer simulations and training sessions in the field.
The US, along with North Korea's neighbours South Korea and Japan, see themselves directly threatened by Pyongyang's nuclear weapons and missile programmes. The one-party state in the north, ruled by Kim Jong Un, considers Washington in particular to be hostile.
North Korea's Defence Ministry on Tuesday accused the US armed forces of preparing an invasion together with South Korea.
North Korea's 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 of wanting to attack the country. They say 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.
