North Korean leader Kim Jong-un oversees military drills involving an underwater drone capable of delivering nuclear attacks in this photo released by the Korean Central News Agency. (KCNA-Yonhap)

North Korea did not answer the daily routine calls South Korea makes using its military and nonmilitary hotlines -- another sign that inter-Korean tensions have been escalating quickly as the North accuses the South and its biggest ally, the US, of ramping up tensions with their annual military drills.

The Unification Ministry in Seoul, which handles the nonmilitary lines, confirmed Friday that Pyongyang had not picked up the calls, made once each in the morning and afternoon, saying assessment of the situation is underway. The calls are meant to check in on and prevent unwarranted spillovers from potential skirmishes -- the chances for which have risen recently.

The Defense Ministry in Seoul, which oversees the military channels, said the same morning and afternoon calls had also gone unanswered, with the North showing no response to the South’s outreach. The ministry did not immediately offer to explain what might have led to the disruption.

A suspension, however, is not unusual for the two Koreas. The North cut off the lines in June 2020, when it destroyed an inter-Korean liaison office to protest anti-North Korea leaflets that had crossed the inter-Korean border. In July 2021, the North resumed communication only to halt it the following month of the same year, blaming it on the regular South-US military drills, which had been suspended or scaled down since 2018.

The drills --“rehearsals for war” to Pyongyang and “tests for readiness” to Seoul and Washington -- are the single biggest point of disagreement putting the two Koreas at odds as they fight over what to do first, between easing international sanctions on North Korea’s nuclear and missile programs and dismantling the programs themselves.

Last month, North Korean leader Kim Jong-un publicly spoke of an “exponentially bigger” nuclear arsenal, having shown off his latest weapons, which can deliver nuclear attacks “anytime and anywhere.” Nuclear attacks are a reality if South Korea and the US do not suspend their field maneuvers, according to the 39-year-old leader.

Analysts expect higher inter-Korea tensions for some time. North Koreans celebrate the birth of late founder Kim Il-sung on April 15, the “Day of the Sun,” and to a lesser extent July 27, when they commemorate the 1950-53 Korean War truce.