North Korea has accused the United States of illegally flying over the economic zone eight times and warned of counter-action.

North Korean leader Kim Jong Un's sister Kim Yo Jong, a key ruling party official, said, in case of repeated illegal intrusion, the US forces will experience a very critical flight.

The Pentagon earlier set aside Pyongyang's accusations of airspace violations and said the U.S. military had adhered to international law.

Kim accused the U.S. Air Force of intruding into the North's "economic water zone" on Monday off the east coast of the Korean peninsula in the sky above the sea 435 km east of Tongchon of Gangwon Province.

U.S. State Department spokesperson Matthew Miller urged North Korea to refrain from escalatory actions and reiterated a call for it to engage in serious and sustained diplomacy.