Hong Kong(CNN) The presence of three US aircraft carriers off the Korean Peninsula is pushing the region closer to nuclear war, according to a North Korean letter to the UN secretary-general.

The three carriers, the USS Ronald Reagan, Theodore Roosevelt and Nimitz, and their multiship strike groups are finishing up four days of exercises Tuesday in the waters between South Korea and Japan.

It is the first time three of the 100,000-ton behemoths have sailed together in the Western Pacific in a decade. South Korean and Japanese warships have joined them during the exercise.

B-1B bombers and F/A-18 strike fighters fly over a Japanese carrier and the USS Theodore Roosevelt.

The exercises, which also have involved US B-1 bombers, help to constitute "the worst ever situation prevailing in and around the Korean Peninsula," Ja Song Nam, North Korea's ambassador to the United Nations, said in a letter Monday to the secretary-general.

The USS Ronald Reagan, Theodore Roosevelt and Nimitz and their strike groups conduct operations.

Washington "is now running amok for war exercises by introducing nuclear war equipment in and around the Korean Peninsula, thereby proving that the US itself is the major offender of the escalation of tension and undermining of the peace," the letter read.

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

Three US carriers have not operated together in the Pacific in a decade.

From the US standpoint, the presence of the three-carrier flotilla in Pacific waters is seen as a signal to North Korea that the United States will not be intimidated by Pyongyang's testing of nuclear weapons and ballistic missiles.

"We sent three of the largest aircraft carriers in the world (to the Korean Peninsula), and a nuclear submarine is also positioned," President Donald Trump said after his arrival last week in South Korea.

During his trip, Trump struck a tough tone on North Korea, calling for an end to its nuclear program and saying "we hope to God we never have to use" the military strength the United States has on the Korean Peninsula.

"This exercise in the Western Pacific is a strong testament to the US Pacific Fleet's unique ability and ironclad commitment to the continued security and stability of the region," Adm. Scott Swift, the commander of the US Pacific Fleet, said in a statement.

Three F/A-18E Super Hornets lead a flotilla of US and South Korea ships in exercises Sunday.

But Pyongyang sees any US military exercises as a threat.

The state-run Korean Central News Agency said Saturday that the presence of the three carriers was part of Washington's "sinister intention to maintain military hegemony in the region."

An F/A-18E Super Hornet launches from the flight deck of the USS Ronald Reagan.

KCNA also cited other recent US military moves, such as the first deployment of stealthy US Air Force F-35 fighters to Okinawa, as signs Washington is ramping up for war against North Korea.

Pyongyang accused the United States and its allies, including Japan, of trying to "escalate the military tension and increase arms race in the region with each passing day."

An F/A-18E Super Hornet launches from the flight deck of the USS Theodore Roosevelt.

North Korean government officials told CNN's Will Ripley in Pyongyang last week that the United States was increasingly taking action that could "ignite another Korean War."

"Nobody knows when and how the 'war maniac' Trump will ignite the 'wick of war,' " the officials said, referring to the presence of the carriers near the peninsula.

A South Korean Defense Ministry official said the three US carriers held drills on either side of the waters separating the Korean Peninsula and Japan. Exercises closest to the Korean coast involved US and South Korean ships, while those closest to Japan involved US and Japanese ships, the official said.

Japan's Maritime Self-Defense Force helicopter destroyer JS Ise sails alongside the USS Nimitz.

Joining the three US carriers and their strike groups were three Japanese Maritime Self-Defense Force warships including the helicopter destroyer JS Ise, a guided-missile destroyer and general purpose destroyer.

"The JMSDF is always using every opportunity to strengthen the JMSDF-USN alliance, and joining an exercise like this is a natural event to stabilize regional security," a Maritime Self-Defense Force statement said.

South Korean vessels joined exercises with the US warships on Sunday, according to US Navy photos.

The South Korean Defense Ministry said it sent two destroyers equipped with the Aegis anti-ballistic missile defense system as well as five other warships to the exercises.

Each US aircraft carrier holds more than 60 aircraft, including F/A-18 fighter jets, airborne early warning and control aircraft and helicopters for anti-submarine warfare.

The Japanese Maritime Self-Defense Force warships join in the Pacific exercises.

The ships accompanying the carriers included destroyers and cruisers equipped with the Aegis system as well as Tomahawk cruise missiles. US attack submarines typically accompany US aircraft carrier strike groups, but the Navy usually keeps their specific locations secret.

The last time three US aircraft carrier strike groups worked together in the Western Pacific was off the US island of Guam in 2007, according to the Navy.