North Korean residents in Gaepung County, North Hwanghae Province are seen from the unification observatory in Paju, South Korea, near the border with North Korea, Aug. 10. Yonhap

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 U.S. 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.