Seoul’s Unification Ministry said North Korea had sent it an official notification to the effect earlier in the day.

“South Korea’s government deeply regrets the entry ban and urges it be lifted immediately,” a ministry spokesman told reporters as quoted by Yonhap, adding that Seoul is doing everything possible to ensure the safety of South Koreans at the joint site.

North Korea pledged on Tuesday to restart facilities at its Yongbyon nuclear complex, including a uranium enrichment plant and a reactor, in a move likely to further ratchet up regional tensions.

The reactor at Yongbyon, which supplied plutonium for North Korea's nuclear weapons program, was shut down in July 2007 as part of international disarmament talks that have since stalled. Analysts say the reactor can produce around 7 kilograms of plutonium a year, or enough for an atomic bomb every twelve months.

The move came after North Korean leader Kim Jong-un said Sunday developing the country’s nuclear weapons was a top priority and a means of maintaining the fragile peace in the region.

Tensions began to rise on the Korean Peninsula after international sanctions were imposed on North Korea in response to a long-range rocket launch in December that world powers condemned as a ballistic missile test. North Korea responded by carrying out a third nuclear test in February, which was followed by more sanctions.

North Korea threatened pre-emptive nuclear strikes on the US mainland and US military bases in the region last month. The threats came as US and South Korean forces carried out annual joint military drills, including near the maritime border between the two Koreas. The United States responded by deploying F-22 Raptor stealth fighters to the region.