Seoul, Apr 18 (IANS): South Korea's Unification Ministry warned North Korea on Tuesday against its unauthorised use of a now-shuttered joint industrial park, saying it will take necessary measures even if it takes time.

The Ministry's call came amid a media report that the North has resumed the operation of some 30 factories at the Kaesong Industrial Complex in the North's namesake border city without permit, reports Yonhap News Agency.

"It is true that more North Korean workers are commuting, compared with the past," a Ministry official told reporters, without elaborating on the details.

The official said the government is "thoroughly" reviewing legal actions against the North's infringement on South Korean companies' property rights.

"Even if it takes time, the government will hold the North accountable for (its unlawful) act and demand necessary compensation," he added.

Last week, Unification Minister Kwon Young-se voiced "strong" regret over the North's unlawful use of South Korean assets left behind in the factory zone, and vowed to take "all possible measures" to hold the secretive regime accountable.

Signs of the North's unauthorised operation of the industrial complex have continued to be detected.

South Korean commuter buses used to transport the North's workers to the factory zone before its 2016 closure were seen operating in the streets of Pyongyang and Kaesong in the latest photos and video footage carried by the North's state media.

South Korea shut down the joint industrial park, once a symbol of inter-Korean reconciliation, in February 2016 in response to North Korea's nuclear and long-range missile tests.