UNITED NATIONS (Reuters) - North Korea complained to the United Nations on Monday about joint military exercises by the United State and South Korea, describing it as  the worst ever situation  because U.S. nuclear war equipment had been deployed ready to strike. In a letter to U.N. Secretary-General Antonio Guterres, seen by Reuters, North Korean U.N. Ambassador Ja Song Nam said the United States was  running amok for war exercises by introducing nuclear war equipment in and around the Korean Peninsula.  Three U.S. aircraft carrier strike groups have been involved in the joint exercise in the Western Pacific in a rare show of force as President Donald Trump visits Asia. The last time three U.S. carrier strike groups exercised together in the Western Pacific was in 2007. South Korea has said the joint drill, due to finish on Tuesday, was in response to North Korean nuclear and missile provocations and to show any such developments by Pyongyang can be repelled with  overwhelming force.  However, Ja said Washington was to blame for escalating tensions and accused the U.N. Security Council of ignoring  the nuclear war exercises of the United States who is hell bent on bringing catastrophic disaster to humanity.  Ja asked Guterres to bring to the attention of the 15-member council - under the rarely used Article 99 of the U.N. Charter -  the danger being posed by the U.S. nuclear war exercises, which are clearly threats to international peace and security.  Tensions have soared between the United States and North Korea following a series of weapons tests by Pyongyang and a string of increasingly bellicose exchanges between Trump and North Korean leader Kim Jong Un. Trump said in a tweet on Sunday that Kim had insulted him by calling him  old  and said he would never call the North Korean leader  short and fat.  The United States has said that all options, including military, are on the table to deal with North Korea, although its preference is for a diplomatic solution. The U.N. Security Council has unanimously ratcheted up sanctions on North Korea over its nuclear and ballistic missile programs since 2006. 