North Korea is reportedly moving aircraft in an effort to strengthen its defenses after the nation's foreign minister said that President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE's tweets amount to a declaration of war.

South Korean lawmaker Lee Cheol Woo said Tuesday that the North appeared to be bolstering its defenses along its east coast, according to Reuters.

Trump in a tweet late Saturday wrote that North Korea "won't be around much longer." His tweet came after North Korean Foreign Minister Ri Yong Ho defended Pyongyang's nuclear and missile development programs during a speech before the United Nations General Assembly.

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Ri on Monday responded to Trump's apparent threat, telling reporters in New York that the president's tweet "declared war" on North Korea. Ri also warned that Pyongyang would shoot down U.S. aircraft, even if they are not in North Korean air space.

"Since the United States declared war on our country, we will have every right to make all self-defensive counter measures, including the right to shoot down the United States strategic bombers at any time even when they are not yet inside the aerospace border of our country," he said.

The U.S. military flew B-1B bombers and F-15 fighter jets near North Korea over the weekend in a show of strength to Pyongyang.

White House press secretary Sarah Huckabee Sanders on Monday rejected the notion that the U.S. had declared war on North Korea, calling Ri's statement "absurd."

Still, the back-and-forth between Washington and Pyongyang has stirred worry among other countries in the region, including China, whose government has warned against a confrontation between the two countries.

North Korea has long sought to develop nuclear weapons and ballistic missiles capable of striking the U.S. But the pace of its weapons testing has accelerated in recent months.

It was revealed this summer that North Korea had developed a miniaturized warhead capable of fitting inside one of its intercontinental ballistic missiles. And earlier this month, the country tested what it claimed was a hydrogen bomb — a development that, if true, would mark a significant milestone for the North's nuclear weapons program.