SEOUL, Mar 4 (AFP)- North Korea on Saturday blamed the United States for what it said was the collapse of international arms control systems and said Pyongyang's nuclear weapons were a just response to ensure the balance of power in the region.

"The Korean peninsula is turning into the world's biggest powderkeg and war practice field due to a military expansion scheme led by the United States and its followers," its foreign ministry said in a commentary carried by state news agency KCNA.

Recent moves by South Korea and Japan showed that a military build-up by the United States and its followers was crossing the danger line and this could not be tolerated, it added.

The comments follow the United States and South Korea's announcement on Friday that they plan to conduct large scale military exercises from March 13-23 to strengthen the allies' combined defensive posture, including focusing on what they called North Korean aggression.

North Korea says such joint military exercises are proof that the United States and its allies are hostile and bent on regime change in the North.