US warships are carrying out training exercises close to the coast of North Korea along with the Japanese military for the first time in 20 years.

But officials have claimed they are not attempting to intimidate Kim Jong-un's secretive state, amid warnings from China over provocation.

The exercise came just two days after The Pentagon tested its missile capability by intercepting a fake nuclear bomb over the Pacific Ocean in response to claims the communist state is building its own long-range weapons, according to Daily Mail.

The two US aircraft carriers - the USS Carl Vinson and the USS Ronald Reagan - are taking part in three days of joint exercises, which began yesterday.

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It is the first such wargames held since the 1990s.

David Helvey, senior adviser on Asia policy to US Defense Secretary Jim Mattis, told reporters that the dual carrier exercise is not intended as a provocation.

He called it routine but acknowledged that it is the first of its kind in about 20 years. The exercises are intended to reassure allies, he said, and to keep US forces ready for any crisis.

"This is not about sending a message directly to North Korea," Helvey said, adding, "I don't expect this to change North Korea's behavior."

In line with Mattis's emphasis on helping allies defend their own territory, the US has deployed a missile defense system in South Korea known as a Terminal High-Altitude Area Defense system.

It is intended to protect South Korea from a potential North Korean missile strike.

Japan Air Self-Defense Force's F-15 fighter jets fly over U.S. Navy aircraft carriers, USS Ronald Reagan, right bottom, and USS Carl Vinson, right top. Photo/AP
Japan Air Self-Defense Force's F-15 fighter jets fly over U.S. Navy aircraft carriers, USS Ronald Reagan, right bottom, and USS Carl Vinson, right top. Photo/AP

The new South Korean government has complained that it was not aware of the extent of THAAD deployments on its territory in recent weeks, but Helvey said the US had consulted with Seoul 'throughout this process' of deploying the THAAD.

The exercises come days after the US Pacific Command reiterated its "ironclad commitment" to protecting allies in South Korea in Japan following the launch of a short-range ballistic missile by Kim's regime.

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In April President Donald Trump declared that the US was sending an "armada" to the Peninsula, in a statement which angered Pyongyang.

It subsequently emerged that the USS Carl Vinson, a key component in Trump's fleet, was actually carrying out exercises with the Australian navy in the Indian Ocean thousands of miles away.

The error emerged after the US Navy posted an image of the aircraft carrier in the Sunda Strait.

Tensions are high in the Korean Peninsula, with the North this week accusing the US of conducting "a nuclear bomb dropping drill".

Japanese Prime Minister Shinzo Abe has said he intends to work with the US to deter North Korea.

But China has urged restraint, calling on both sides not to engage in provocation.

The Carl Vinson has been on high alert since April after missile launches by the secretive state, and has already taken part in drills with South Korean navy ships in waters off the Peninsula.

It has now been joined by the USS Ronald Reagan, which is back in operation following maintenance last month.

The training exercises with the Japanese navy are believed to be scheduled to last several days.

Washington described Thursday's drill as "routine training".

This image released by the US Navy shows the USS Ronald Reagan (front) and USS Carl Vinson (right, at the back) carrying out drills in the Sea of Japan. Photo/AP
This image released by the US Navy shows the USS Ronald Reagan (front) and USS Carl Vinson (right, at the back) carrying out drills in the Sea of Japan. Photo/AP

"The Ronald Reagan and Carl Vinson Strike Groups were joined by the Japan Maritime Self-Defense Forces (JMSDF) for routine training to improve interoperability and readiness in the Indo-Asia Pacific," the US Seventh Fleet said on its Facebook page.

Japanese military officials declined to comment on the drill.

Navy spokesman Lt. Loren Terry told VOA that the operations in international waters have been designed to train commanders to respond to "regional situations".

He added: "This unique capability is one of many ways the U.S. Navy promotes security, stability and prosperity throughout the Indo-Asia-Pacific."

It is the first time in decades that two aircraft carriers have carried out drills close to North Korea, the US Naval Institute states.

The United States currently has 28,500 troops stationed in South Korea.

The countries are bound by a mutual defense treaty dating back to the end of the 1950-53 Korean War, which ended in a truce that has left the peninsula in a technical state of war.

South Korea's defence ministry said on Tuesday that it had conducted a joint drill with a US supersonic B-1B Lancer bomber the previous day.

North Korea's state media had earlier accused the United States of staging "a nuclear bomb dropping drill".

The North's KCNA news agency reported on Tuesday that Kim had supervised the test of missile equipped with a new precision guidance system and an improved pre-launch automated sequence and a new mobile launch vehicle.

The USS Carl Vinson (front) and USS Ronald Reagan are carrying out joint training in the Sea of Japan yesterday. Photo/AP
The USS Carl Vinson (front) and USS Ronald Reagan are carrying out joint training in the Sea of Japan yesterday. Photo/AP

Kim said North Korea would develop more powerful weapons in multiple phases in accordance with its timetable to defend North Korea against the United States.

"He expressed the conviction that it would make a greater leap forward in this spirit to send a bigger 'gift package' to the Yankees" in retaliation for American military provocation, KCNA quoted Kim as saying.

On Tuesday, The Pentagon successfully tested a U.S. long-range interceptor missile over the Pacific Ocean.

The exercise was aimed at seeing how ready American forces would be to counter a potential threat from North Korea.

Chinese Ministry of Foreign Affairs spokesman Hua Chunying said: "We have all been following how the situation on the Korean Peninsula evolves over the recent period of time, and hoping that tensions can be lowered, and that the nuclear issue of the Korean Peninsula can be brought back to the track of dialogue and consultation as soon as possible.

"The situation on the Peninsula is highly complex and sensitive. There is a window of opportunity for us to bring the nuclear issue back to the right track of dialogue and consultation.

"In this connection, we hope that all relevant parties would exercise restraint and do more to ease the tension and build up mutual trust, rather than provoke each other and escalate the tension."