North Korea has warned that any moves by the US to shoot down its test missiles will be considered a "declaration of war".
Kim Yo-jong has warned the US about shooting down North Korean missiles
Kim Yo-jong, the sister of leader Kim Jong-un, has blamed a joint military exercise between the US and South Korea for the growing tensions.
According to state media, she also hinted that Pyongyang could fire more missiles into the Pacific Ocean.
The US and its allies have never shot down any North Korean ballistic missiles - which are banned by the UN Security Council - but the question has drawn additional scrutiny since Kim Jong-un's regime suggested that it would fire more missiles over Japan.
Kim said: "The Pacific Ocean does not belong to the dominium of the US or Japan."
Analysts have warned that North Korea follows through on its threats to continue firing missiles, it would be able to make technical advances in addition to signaling its military resolve.
In a separate statement, the chief of the Foreign News Section at North Korea's Foreign Ministry accused the US of "aggravating" the situation by conducting a joint air drill and planning field exercises with South Korea.
South Korea's Unification Ministry, which handles relations with its neighbour, responded by suggesting that Pyongyang's "reckless nuclear and missile development" is to blame for the situation.