North Korea is not responding to regular contact through inter-Korean communication lines for the third day.

Seoul's defense ministry said Sunday that the North did not answer the 9 a.m. opening call via the east and west sea military hotline.

Pyongyang was unresponsive to the military channel as well as that of the inter-Korean liaison office on Friday and has yet to give any reason for the lack of response.

The two Koreas hold regular phone calls twice a day, in the morning and afternoon, via the joint liaison office as well as their military channel.

While the liaison office operates only on weekdays, the military hotline remains open on weekends as well for emergency purposes.

It is unclear why North Korean officials are refusing to answer the calls.

Ministry officials said that they plan to try to call the North at 4 p.m.