New York/Seoul, Sept. 25 (Reuters): North Korea's foreign minister said today that President Donald Trump had declared war on North Korea and that Pyongyang reserves the right to take countermeasures, including shooting down US bombers even if they are not in its air space.

Ri Yong-ho said a Twitter message by Trump on Saturday in which the President warned that the minister and North Korean leader Kim Jong-un "won't be around much longer" if they acted on their threats, amounted to a declaration of war.

"The whole world should clearly remember it was the US who first declared war on our country," Ri told reporters in New York.

"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 said. "The question of who won't be around much longer will be answered then," Ri said.

On Saturday, US Air Force B-1B Lancer bombers escorted by fighters flew in international airspace over waters east of North Korea in a show of force after a heated exchange of rhetoric between Trump and Kim over North Korea's nuclear and missile programmes.

The Pentagon said the flight was the farthest north of the demilitarised zone separating North and South Korea that any US fighter jet or bomber has flown in the 21st century.

"That operation was conducted in international airspace, over international waters, so we have the right to fly, sail and operate where legally permissible around the globe," Pentagon spokesman Col Robert Manning said today.

North Korea, which has remained technically at war with the US since the 1950-53 Korean War ended in a truce not a peace treaty, has been working to develop nuclear-tipped missiles capable of hitting the US mainland and conducted its sixth and largest nuclear test this month.

UN secretary-general Antonio Guterres said today that the only solution to the crisis was a political one. "Fiery talk can lead to fatal misunderstandings," UN spokesman Stephane Dujarric told reporters.

China, North Korea's neighbour and main ally, which has nevertheless backed UN sanctions over Pyongyang's nuclear programme, called today for all sides in the crisis to show restraint and not "add oil to the flames". China's foreign ministry spokesman Lu Kang described the situation as highly complex and sensitive. He said it was vitally important everyone strictly, fully and correctly implemented all North Korea related UN resolutions.

All sides should "not further irritate each other and add oil to the flames of the tense situation on the peninsula at present", Lu said.

Speaking to British Prime Minister Theresa May by telephone, Chinese President Xi Jinping repeated Beijing's position that the North Korean issue should be resolved peacefully via talks and hoped Britain could play a constructive role.