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

The North reportedly did not answer the 9 a.m. opening call on Wednesday via the East and West Sea military hotlines as well as that of the inter-Korean liaison office.

Pyongyang has been unresponsive to routine calls since it stopped answering last Friday, and has yet to give any reason for the apparent refusal to respond.

South Korean unification minister Kwon Young-se issued a rare statement on Tuesday, voicing strong regret for the refusal to answer.

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.

Although no official explanation has been given for the lack of response from North Korea, speculation is focusing on the likelihood that the North is not answering the calls in protest against joint military exercises between Seoul and Washington.