The chilling sound of a nuclear warning siren echoed in Hawaii on Friday, renewing a monthly exercise that has not been in place since the end of the Cold War.

The decision to bring the system back was triggered by escalating threats between the President Donald Trump and North Korean leader Kim Jong Un. While decades of international collaborations have tried to stunt North Korea’s weapons program, the country this past year has made rapid progress, claiming to have tested its biggest nuclear weapon in September in a direct challenge to Trump.

Hawaii officials have told the Washington Post that they believe the chance that an attack on their state is low, but in the event of one, the sirens will warn people that they have 15 minutes to connect with loved ones and take cover.

“I told my 13-year-old granddaughter, ‘If you hear the siren you take out your phone, you call your mother and you talk to her for 15 minutes,” Laura Chang, a 67-year-old custodian, told the Washington Post. “‘Go underneath the desk, call your mother, and know that I love you.’”

Here is Hawaii testing its nuclear attack warning siren from Ala Moana Beach Park in Honolulu. pic.twitter.com/kh9EPElvNL — Honolulu Civil Beat (@CivilBeat) December 1, 2017

Kim Jong Un and Trump’s relationship has been a focus of international attention since Trump took office. While North Korea is testing weapons that it claims can reach anywhere, Trump, in defiance of his advisers, has threatened major sanctions and blacklisted the country as a state sponsor of terrorism.

North Korea has defied all previous rounds of sanctions, testing 20 missiles this year alone, according to the Washington Post.

And in September, the country claimed to have carried out its sixth nuclear test since 2006 — the first during Trump’s presidency — to “examine and confirm the accuracy and credibility” of North Korea’s technology, according to the Korean Central News Agency. It was estimated to be about four to five times stronger than what the U.S. dropped on Japan’s Nagasaki.

The test “marked a very significant occasion in attaining the final goal of completing the state nuclear force,” according to the agency, because it can be detonated at high attitudes and if attached to a missile could reach the U.S.

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Trump, who has previously threatened to “totally destroy” North Korea, told the United Nations that Kim Jong Un was on a “suicide mission,” using bold language that has some analysts worried and others satisfied.

Kim Jong Un responded with a rare personal statement, saying he saw Trump’s remarks as a declaration of war.

On Wednesday, North Korea tested an intercontinental ballistic missile that flew 500 miles higher than previous missile tests of its kind, claiming it is strong enough to carry a nuclear warhead to the U.S.

Trump responded in a tweet, “Additional major sanctions will be imposed on North Korea today. This situation will be handled!”

But officials in Hawaii on Friday talked about the necessity for precautions.

Kurt Leong, a Kauai Island fire captain, told the Washington Post how important public awareness is to Hawaii, a state of about 1.4 million people that reels in 8 million tourists a year.

“We were talking about it this morning at the firehouse, and we don’t know exactly what the proper protocol is,” Leong said. “I do think the federal government and the state and the counties have a long way to go in terms of getting that information out there to the public. It’s not like it’s pasted everywhere about what to do.”

The Attack Warning Tone, which sounds like the slowed wailing of an ambulance, will continue to ring on the first business day of every month after another regular test for hurricanes and tsunamis.

The last time it sounded was in the mid-1990s, after the Cold War.

“At that point there was no longer a need for it. Somebody finally decided we don’t need this anymore. It was not really serving a purpose,” Richard Rapoza, the spokesman for the emergency management agency, told The New York Times. “Now, because of the North Korea threat we are starting to use it again.”