Seoul: South Korea threatened to take "necessary steps" on Thursday if Pyongyang kept using a joint industrial complex in the North that was once seen as a symbol of peace without permission. The Kaesong Industrial Complex used to employ more than 50,000 North Korean workers who produced goods like watches and clothing for about 125 South Korean businesses that provided the funding and machinery. In response to the North's nuclear test and missile launches in 2016, Seoul withdrew from the project that had been set up in the wake of the 2000 inter-Korean summit, claiming that Kaesong profits were financing the provocations. Also Read: Due to rising inflation, this country gave work-up homes to employees, but... However, the unification ministry in Seoul said Thursday that the North has continued to use the facility and its South Korean-owned assets without authorization. North Korea rejected a notice demanding that activity at the factory complex stop sent by South Korea's liaison office on Thursday, the ministry said. The announcement was made a day after a set of images published by the North Korean newspaper Rodong Sinmun appeared to show a South Korean bus that had once been used to transport employees to a complex operating in Pyongyang. Also Read: Japan, the US, and South Korea demand support for the ban on hiring North Koreans According to Seoul's notice and the two Koreas' agreement on investment guarantees, the facilities' continued use "regardless of the will of the businessmen is a clear violation of property rights." "North Korea needs to stop doing this right away. We will take the necessary actions assuming North Korea has admitted to using the complex without permission if it does not respond to our request in a timely manner, the notice said. There was no mention of the potential course of action. The business park, which provided the impoverished North with valuable hard currency as well as cheap labour and tax breaks for the participating companies, was essentially the last form of economic cooperation between the Koreas while it was still operational. Since the 2019 Hanoi summit between leader Kim Jong Un and the US president in office, Donald Trump, failed to produce any concrete results, North Korea has intensified efforts to expand its military capabilities and nuclear programme. Also Read: Six suspects are detained in Azerbaijan for a "coup plot" It has conducted the launch of an intercontinental ballistic missile in recent weeks, according to state media, and tested what it claimed to be an underwater drone with nuclear capability. Since the beginning of 2022, Seoul and Washington officials have issued warnings that North Korea may be gearing up to conduct its seventh nuclear test; some experts even believe the test may be just around the corner.