South Korea on Wednesday urged the Democratic People's Republic of Korea (DPRK) to normalize the passage of South Korean workers to the Kaesong Industrial Complex after the northern neighbor banned South Korean workers' entrance to the inter-Korean industrial park located at the DPRK's border town of Kaesong.

"The entry ban to the Kaesong industrial complex is causing a serious barrier to stable operation of the complex," Kim Hyung- seok, spokesman of the Ministry of Unification told reporters at a briefing. "The government is urging once again North Korea (DPRK)'s immediate normalization of the entry ban, while expressing strong regrets over the ban."

The comments came shortly after the DPRK banned South Korean workers' entrance to the joint industrial zone between Seoul and Pyongyang earlier in the day, only allowing the workers to leave Kaesong to come back to Seoul.

The entry ban came four days after Pyongyang threatened to shut down the industrial park, which is housing around 120 South Korean companies and is employing some 54,000 DPRK workers.

Despite the normal operation of factories at the industrial zone until now, the entry ban would stop the supply of additional raw materials and workers to the plants, which may lead to a disruption in production if the ban is prolonged.

The DPRK said on Tuesday that it will restart operations at the Yongbyon nuclear facilities, including a uranium enrichment plant and a five-megawatt graphite moderated reactor that had been " mothballed and disabled" in October 2007 under an agreement reached at the six-party talks.

Tensions have been running high on the Korean Peninsula since the DPRK conducted its third nuclear test on Feb. 12 in a protest against the joint military drills between Seoul and Washington.

The DPRK declared last weekend that it had entered "a state of war" against South Korea, saying that the state of neither peace nor war has ended on the Korean Peninsula. It has also threatened to launch a preemptive nuclear strike for self-defense, unilaterally nullifying the 1953 armistice that ended the 1950-53 Korean War.

In response to the DPRK's repeated threats of war, US Strategic Command dispatched two B-2 stealth bombers that can carry nuclear weapons from Missouri to conduct its first-ever firing drill in South Korea. It also positioned F-22 stealth fighters for the ongoing joint military drill between Seoul and Washington.

South Korean President Park Geun-hye on Monday instructed the military to take a strong response to possible provocations by the DPRK without any political consideration, while US Secretary of State John Kerry on Tuesday reaffirmed its defense commitment to South Korea after meeting with his South Korean counterpart Yun Byung-se.

Despite the escalating tensions, it will stop short of undermining the credit fundamentals of South Korea, the US credit rating appraiser said.

"North Korea (DPRK)'s rhetoric highlights long-standing geopolitical event risk which, however, has not had material, adverse affects on market sentiment in Seoul or on South Korea's economic fundamentals," Thomas Byrne, senior vice president at Mooyd's Investors Service, said in a report.

Byrne noted that South Korea's stability has been ensured by the deterrence provided by its robust alliance with the US.