North Korea has not responded to regularly held calls from South Korea since late last week, Seoul said Tuesday, as speculation grows over further possible provocations by Pyongyang, including a nuclear test.

"The government expresses strong regret over the North's unilateral and irresponsible attitude," South Korean Unification Minister Kwon Young Se said in a statement.

South Korean Unification Minister Kwon Young Se gives a statement in Seoul on April 11, 2023, in which he expressed disappointment over North Korea's unresponsiveness to routine inter-Korean calls. (Kyodo) ==Kyodo

"We strongly warn that it will only result in isolation of North Korea itself, making (the country) face more difficult situations."

North Korea has not answered phone calls through inter-Korean liaison lines and military hotlines since Friday, according to the Ministry of Unification.

Kwon also condemned North Korea, saying it has been using South Korean facilities against Seoul's wishes in the Kaesong Industrial Complex in the southwestern part of North Korea, where the two countries have conducted economic cooperation projects.

Earlier in the day, the North's state-run media said the country's leader Kim Jong Un has stressed the need to expand Pyongyang's war deterrence in a "more practical and offensive" manner and "effectively apply" it for stricter control and management of the ever-worsening security situation on the Korean Peninsula.

Kim made the remarks Monday at a meeting of the Central Military Commission of the ruling Workers' Party of Korea, which examined a "plan for offensive operation on the front," the North's official Korean Central News Agency said without elaborating.

The meeting also discussed practical matters and measures for machinery to "prepare various military action proposals that no means and ways of counteraction are available to the enemy" amid the "escalating moves" of the United States and South Korea to "unleash a war of aggression," KCNA said.

A picture released by KCNA showed Kim pointing his finger at what appears to be a map of the Korean Peninsula, though the image was blurred.

South Korea's Yonhap News Agency said it possibly indicated that the North's weapons could put the South within operational range.

Some observers said Kim seems to be pointing to areas around a key U.S. military base in Pyeongtaek, 65 kilometers south of Seoul, according to Yonhap.

Kim's call for bolstering the North's war deterrence has raised concerns in the region, with Japanese Chief Cabinet Secretary Hirokazu Matsuno saying Tuesday that Pyongyang may carry out further provocative actions, including missile launches and a nuclear test.

Speculation has been growing that the North could soon conduct its seventh nuclear test and first since September 2017.

Related coverage:

North Korea conducts another test of underwater nuclear attack drone

U.S. diplomat sees opportunity in renewed Japan-S. Korea cooperation

South Korea, Japan, U.S. call for deportation of North Korean workers