North Korea did not answer South Korea's phone calls via cross-border communication lines for the third straight day Thursday, officials said, in apparent protest against the summertime military exercise between the South and the United States.

The morning calls via the inter-Korean liaison office and military communication channels in the eastern and western border regions went unanswered earlier in the day, according to the officials.

North Korea began to shun picking up regular calls late Tuesday as South Korea and the US kicked off a four-day preliminary training in the runup to the main combined exercise next week despite the North's warning it will cloud inter-Korean relations.

Since the inter-Korean hotlines were restored late last month following a yearlong severance, calls were made once in the morning and again in the afternoon on a daily basis.

On Wednesday, Kim Yong-chol, a senior North Korean official, issued a statement saying the North will make the South "realize by the minute what a dangerous choice they made and what a serious security crisis they will face because of their wrong choice."

"We are closely monitoring the North's moves under close coordination of South Korean and US intelligence authorities, and maintaining a firm readiness posture," a military officer said, adding that no unusual activities have been detected as of now.

Whether and how to conduct the allies' annual exercise drew keen attention, particularly after North Korean leader Kim Jong-un's sister Kim Yo-jong warned early this month the drills would dampen the conciliatory mood created in the wake of the restoration of the communication lines. (Yonhap)