In this November 12, 2017 photo provided by South Korea Defense Ministry, three USA aircraft carriers USS Theodore Roosevelt, top left, USS Ronald Reagan, top center, and USS Nimitz, top right, participate with South Korean Navy's Aegis destroyer, King Sejong the Great, bottom, during the joint naval exercises between the United States and South Korea in waters off South Korea's eastern coast in South Korea.

Of the three flattops - USS Ronald Reagan, USS Nimitz and USS Theodore Roosevelt - one has already entered the operational area, called the Korea Theater of Operations (KTO), while the others will partake later in the day following drills with Japan, an informed source said.

The exercises show North Korea is right to build up its defenses, the letter said.

He said: "The United States has the world power and unique capability to be able to bring this kind of fire power to bear at a time and place of our choosing".

"But, that peaceful resolution is backed up by having the ability to do what is necessary should that become needed".

The training is expected to showcase the military strength and United States alliance with Japan and South Korea amid the ongoing conflict with North Korea, according to information obtained by Association Press.

Along with the three carrier groups, he said, the U.S. has reactivated round-the-clock sorties with nuclear-capable B-52 strategic bombers "which existed during the Cold War times".

The ambassador added: "The large-scale nuclear war exercises and blackmails, which the U.S. stages for a whole year without a break in collaboration with its followers to stifle our republic, make one conclude that the option we have taken was the right one and we should go along the way to the last".

"The combined drills underscore the strong might and will of the South Korea-U.S. alliance", a Seoul official said, declining to be named.

The exercise is the first time three carrier strike groups have operated together in the Western Pacific since exercises Valiant Shield 2006 and 2007 off the coast of Guam, according to Navy officials.

Traditionally Kim Jong-un's troops train from December to April.

The South Korea conducted the exercise with seven vessels including three destroyers.