North Korea's foreign minister on Monday asserted that the pariah state has the right to defend itself by shooting down US planes, even if they are not in the country's airspace.

Ri Yong Ho, speaking to reporters at a hotel across the street from the United Nations, said US President Donald Trump's comments at the General Assembly last week constituted a declaration of war.

"The whole world should clearly remember it was the US who first declared war on our country," he said. "Since the United States declared war on our country, we will have every right to make countermeasures, including the right to shoot down United States strategic bombers even when they are not inside the airspace border of our country."

Ri's remarks were the most direct and threatening so far since Trump gave a combative address to the General Assembly last week in which he threatened to "totally destroy" North Korea.

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Lauding the "great strength and patience" of the United States, Trump said that "if it is forced to defend ourselves or its allies, we will have no choice but to totally destroy North Korea."

US President Donald Trump address at the 72nd session of the United Nations General Assembly has inflamed tensions with North Korea. Photo / AP
US President Donald Trump address at the 72nd session of the United Nations General Assembly has inflamed tensions with North Korea. Photo / AP

Tensions have escalated almost every day since then. North Korea's leader, Kim Jong Un, made a rare TV appearance in which he called Trump "mentally deranged." Trump responded with mockery, calling Kim "little rocket man."

Ri, who said North Korea was prepared to test a hydrogen bomb over the Pacific Ocean, told the United Nations on Friday that Trump's disrespect toward Kim made it "inevitable" that rockets would "visit" the US mainland.