North Korea warned on Tuesday that it would be a "clear declaration of war" if its missiles were shot down during their test launches over the Pacific Ocean.

The United States and South Korea have ramped up defence cooperation, including joint drills, in the face of growing threats from nuclear-armed Pyongyang, which has conducted a wave of banned weapons tests in recent months.

North Korea has said its nuclear weapons and missile programmes are for self-defence, and has bristled over US-South Korea military exercises, describing them as rehearsals for an invasion.

"It will be regarded as a clear declaration of war against the DPRK, in case such military response as interception takes place against our tests of strategic weapons," Kim Yo Jong, North Korean leader Kim Jong Un's powerful sister, said in a statement, using North Korea's official name.

"The Pacific Ocean does not belong to the dominium of the US or Japan."

North Korea is "always on standby to take appropriate, quick and overwhelming action at any time", added her statement, published by the official Korean Central News Agency.

This month, the US and South Korean militaries will hold their largest joint drills in five years.

Ahead of those exercises, named Freedom Shield and scheduled for at least 10 days starting March 13, the allies held air drills this week featuring a nuclear-capable US B-52 heavy bomber. (AFP)