The rejection could complicate efforts by South Korea's news liberal President Moon Jae-in to try to expand civilian exchanges with North Korea as a way to improve strained bilateral ties.
All major cooperation programs between the rivals remain stalled amid an international standoff over North Korea's nuclear and missile programs.
In late May, Moon's government allowed the Seoul-based Korean Sharing Movement to contact North Korea. It was South Korea's first approval of cross-border civilian exchanges since January 2016.
But North Korea told his organisation today that it won't allow the visit because of UN sanctions adopted last week that Seoul has vowed to implement, Hong said.
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The UN Security Council voted unanimously Friday to add 15 individuals and four entities linked to North Korea's nuclear and missile programs to a UN sanctions blacklist.
It's unclear if and how much the new UN sanctions would sting North Korea, which is already under multiple rounds of UN and other international sanctions.
Moon's government has said it will evaluate expanding civilian exchanges with North Korea, while sternly dealing with its missile and other weapons tests.
North Korea is pushing hard to build a nuclear-tipped missile that can reach the continental United States.