South Korean media report North Korea's foreign minister has said the country may test a hydrogen bomb in the Pacific.

North Korean Foreign Minister Ri Yong Ho said on Friday (US time) he believes the North could consider a hydrogen bomb test on the Pacific Ocean of an unprecedented scale, South Korea's Yonhap news agency reported.

Ri was speaking to reporters in New York when he was asked what North Korean leader Kim Jong-Un had meant when he threatened in an earlier statement the "highest level of hard-line countermeasure in history" against the United States.

Ri told reporters in New York that a response "could be the most powerful detonation of an H-bomb in the Pacific," although he did not know Kim's exact thoughts, Yonhap reported.

Ri reportedly added that "We have no idea about what actions could be taken as it will be ordered by leader Kim Jong Un."

Ri is in New York attending the United Nations General Assembly.

Such a test would be considered a major provocation by Washington and its allies.

Ri's threat is significant because such a detonation would move North Korea's nuclear weapons activities beyond its borders for the first time.

The communist dictatorship's previous nuclear tests have taken place in its isolated mountains.

Hydrogen bombs are more powerful by an order of magnitude than the atomic bombs that North Korea tested in previous years. The country claims that an atomic test it carried out early this month was an H-bomb.

The apparent threat comes after Kim issued a rare statement calling US President Donald Trump "deranged" and warning he "pay dearly" for his threats.

In his maiden speech to the UN two days ago, Trump said he would "totally destroy" North Korea if it attacked the US or one of its allies.

"Far from making remarks of any persuasive power that can be viewed to be helpful to defusing tension, he made unprecedented rude nonsense," Kim said today.

"A frightened dog barks louder," Kim said. "He is unfit to hold the prerogative of supreme command of a country, and he is surely a rogue and a gangster fond of playing with fire."

Pyongyang launched an intercontinental ballistic missile over Japan exactly one week ago, earning a fresh new round of sanctions.

Seoul's spy agency also warned the North could launch an ICBM on a standard trajectory toward the Pacific Ocean around its founding anniversary of the ruling Workers' Party of Korea on October 10.