North Korean leader Kim Jong Un’s New Year's speech. Jung Yeon-Je/AFP/Getty Images US President-elect Donald Trump says he won't let North Korea drop a nuclear bomb on America.

In his latest instance of diplomacy-by-tweet, Trump on Monday said: "North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won't happen!"

North Korean leader Kim Jong Un, in a televised New Year's Day speech, said his country was about to test its first intercontinental ballistic missile, which could be equipped with nuclear warheads and reach the US.

"Research and development of cutting-edge arms equipment is actively progressing, and ICBM rocket test-launch preparation is in its last stage," Kim said during the televised speech.

Trump followed his tweet with another accusing China of not helping to keep North Korea under control.

"China has been taking out massive amounts of money & wealth from the U.S. in totally one-sided trade, but won't help with North Korea," Trump said. "Nice.”

Donald Trump. AP Photo/ Evan Vucci

UN sanctions have been in place against North Korea over its nuclear and ballistic missile tests since 2006.

North Korea increased its testing of ballistic missiles last year, and in September it conducted its fifth and largest nuclear test.

This prompted the UN Security Council to tighten sanctions.

The Pentagon reacted to North Korea's latest announcement by warning the country against "provocative actions."

The US said: "Multiple UN Security Council resolutions explicitly prohibit North Korea's launches using ballistic missile technology."

In a series of tweets last month, Trump accused China of currency manipulation and militarizing the South China Sea.

He also upset China by having a protocol-breaking phone conversation with Taiwanese President Tsai Ing-wen, who called to congratulate Trump on his election win.