North Korea has warned that US-South Korean military exercises — which it called "the most undisguised nuclear war manoeuvrers" — are driving the Korean Peninsula and north-east Asia towards "nuclear disaster".

Key points: North Korea's UN ambassador warns that "it may go over to an actual war"

North Korea's UN ambassador warns that "it may go over to an actual war" The White House announces more defence capabilities to be deployed to the region

The White House announces more defence capabilities to be deployed to the region The raised tensions follow another series of North Korean ballistic missile tests

The raised tensions follow another series of North Korean ballistic missile tests On Monday some 50,000 North Koreans rally in support of mobilisation efforts

North Korea's UN ambassador, Ja Song-nam, said in a letter to the UN Security Council on Monday the US was using nuclear-propelled aircraft carries, nuclear submarines, nuclear strategic bombers and stealth fighters in the joint exercises that began on March 1.

"It may go over to an actual war," he warned of the military drills.

"Consequently, the situation on the Korean Peninsula is again inching to the brink of a nuclear war."

Mr Ja again urged the Security Council to discuss the US-South Korea exercises and warned if it ignored North Korea's request as it had in the past it would demonstrate the UN's most powerful body was only a "political tool" of the United States.

The ambassador said the United States sought to convince public opinion that the joint exercise was a response to North Korea's nuclear weapons, but said the US and South Korea carried out military drills numerous times before Pyongyang possessed its "nuclear deterrent".

North Korea sent the letter on the manoeuvrers hours after North Korea fired four banned ballistic missiles earlier on Monday, in apparent reaction to the US-South Korean exercises.

Three of them landed in waters Japan claims as its exclusive economic zone, South Korean and Japanese officials said.

US President Donald Trump discussed plans to respond to the recent missile launches with South Korea's acting president, Hwang Kyo-ahn, an official at Mr Hwang's office said.

Mr Trump also had a phone call with Japanese Prime Minister Shinzo Abe, reaffirming its alliance with Tokyo and condemning North Korea's tests as a threat to regional security.

THAAD deployment begins

The US started to deploy the first elements of its advanced Terminal High Altitude Area Defence (THAAD) anti-missile system to South Korea in response to the missile tests, US Pacific Command said.

The THAAD missile system — like this one, pictured during a US test — is being deployed to South Korea. ( Reuters )

"Continued provocative actions by North Korea, to include yesterday's launch of multiple missiles, only confirm the prudence of our alliance decision last year to deploy THAAD to South Korea," US Pacific Commander Admiral Harry Harris said in a statement.

South Korea's media cited the nation's military as saying the THAAD deployment would be complete in one to two months and would be operational as early as April.

The move by the US military is likely to deepen the brewing conflict between South Korea and China, which has angrily opposed the THAAD deployment as destroying regional security balance.

Meanwhile, North Korean authorities gathered more than 50,000 people together on Monday to make a show of support for the country's latest mobilisation campaign.

The missile launches come amid pushes by leader Kim Jong-un for a nuclear and missile program. ( Reuters, file )

Mr Ja said the main reason the Democratic People's Republic of Korea (DPRK) — the country's official name — was equipping itself "with nuclear attack capabilities" and strengthening its nuclear deterrent forces was in self-defence against what he called the US "extreme anti-DPRK hostile police and nuclear threats and blackmails as well as manoeuvrers to enforce its nuclear weapons".

North Korea's UN Mission also issued a press statement denouncing and rejecting a report by the Security Council's panel of experts that monitors UN sanctions against the DPRK.

The experts said North Korea was flouting sanctions by trading in prohibited weapons and other goods and using evasion techniques "that are increasing in scale, scope and sophistication".

The DPRK Mission again insisted the sanctions "have no legal basis at all" and violate the country's "lawful rights".

ABC/Wires