North Korea did not answer South Korea's phone calls via their military hotlines Tuesday, the defense ministry said, after the North vowed to sever all communication lines with the South in its first step against what it called an "enemy."

Earlier in the day, North Korea said it will "completely" close the inter-Korean liaison line, as well as hotlines between their militaries and presidential offices, at noon Tuesday in protest against Seoul's failure to stop defectors from sending anti-Pyongyang leaflets into the North.

"North Korea did not answer our calls via the military communication lines this morning," defense ministry spokesperson Choi Hyun-soo said, adding that it is the first time that the North did not respond since the hotlines were restored in 2018.

The defense ministry later said the North also did not answer its second regular phone call Tuesday.

The military authorities of the two Koreas had regular telephone calls twice a day -- at 9 a.m. and 4 p.m. -- via their eastern and western communication lines until Monday, though the North did not pick up the South's morning liaison office call that day.

"We will continue to monitor the situation," Choi said, refusing to reveal details.

The military hotlines are meant to prevent accidental military clashes between the two Koreas. The west coast communication line, in particular, was used to send notices to the North Korean side.

The communist country also did not respond to the South's contact via their ship-to-ship radio links in the day, according to the official. The inter-Korean maritime communication was also restored in 2018 after a decade of being severed.

The severance came after Kim Yo-jong, the powerful sister of leader Kim Jong-un, on Friday threatened to scrap an inter-Korean tension-reduction military agreement signed in 2018 and to halt other inter-Korean projects, such as the liaison office and a joint factory park in the border town of Kaesong, if the South fails to prevent activists from flying anti-Pyongyang leaflets across the border.

The communist country warned that the cut-off is "the first step of the determination to completely shut down all contact means with South Korea and get rid of unnecessary things," vowing to turn its work toward the South "into the one against an enemy," according to the North's official Korean Central News Agency (KCNA) on Tuesday.

The US-led United Nations Command (UNC) has not responded to Yonhap News Agency's request for confirmation as to whether the command maintains its hotline with the North Korean military.

UNC earlier said that it uses the line twice per day for communication checks, and the two sides exchanged over 130 messages last year via the hotline. The command enforces the armistice agreement that halted the 1950-53 Korean War. (Yonhap)