North Korea did not answer South Korea's phone calls via liaison and military hotlines for the second consecutive day Wednesday, officials said, as Pyongyang is ramping up criticism against the South for going ahead with its summertime exercise with the United States.

The calls via the inter-Korean liaison office and military communication channels in the eastern and western border regions went unanswered at 9 a.m., after the North began to shun their regular calls from Tuesday afternoon, according to the officials.

"We are closely monitoring the situation," a military official said.

The disconnection followed a series of angry statements from Pyongyang after South Korea and the US began a four-day preliminary training in the run-up to the main combined exercise set to kick off next week, despite the North's warning it will cloud inter-Korean relations.

Kim Yong-chol, a senior North Korean official, issued a statement earlier in the day, saying that 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."

"They must be made to clearly understand how dearly they have to pay for answering our good faith with hostile acts after letting go the opportunity for improved inter-Korean relations," he said in the statement carried by the North's official Korean Central News Agency.

The inter-Korean communication lines were restored late last month following a yearlong severance after President Moon Jae-in and North Korean leader Kim Jong-un agreed to improve their chilled ties amid little progress in nuclear negotiations.

On Tuesday, Kim Yo-jong, the sister of Kim Jong-un, bristled at South Korea and the US over the exercise, vowing to strengthen the country's national defense and preemptive strike capabilities "for rapidly countering any military actions against us."

Whether and how to conduct the allies' annual exercise drew keen attention, particularly after 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.

Sources have said the South decided to go ahead with the exercise in a scaled-back manner, but Kim said it is still a "war rehearsal and preliminary nuclear war exercise" regardless of its scale or mode.

Following Kim's statement, Ned Price, a US State Department spokesman, said the US "harbors no hostile intent towards" North Korea, stressing that the drills are "purely defensive in nature." (Yonhap)