In this April 29, 2016, file photo, members of a South Korean civic group send leaflets denouncing the North Korean regime in Tanhyeon, Paju, near the North Korean border. Korea Times photo by Choi Won-suk

The sending of leaflets into North Korea by defectors is an exercise of the right to freedom of expression, a U.N. official said, amid Pyongyang's threats to punish Seoul for failing to stop the launches that criticize the North Korean leader.

Signe Poulsen, head of the Seoul office of the U.N. Office of High Commissioner for Human Rights, made the remarks amid heightening tensions on the Korean Peninsula after the North threatened to retaliate against the South for what it called Seoul's "connivance" at the floating of anti-Pyongyang materials in large balloons.

The flying of leaflets has long been a source of tensions between the two Koreas, as the leaflets contain strong criticism of the North's authoritarian regime and leader Kim Jong-un, as well as the country's poor human rights situation.

"It's very difficult to give information to people in North Korea. There are organizations who're trying to reach North Koreans by flying balloons, there are some who try to get information through radios. I think their intentions are very genuine ones," she said in an interview with Yonhap News Agency on Monday.

"I think, escapees who've come from North Korea themselves, know more about that than me or any kind of so-called North Korea experts," she said.

The North has revealed plans to revert to treating the South like an enemy and to send its own propaganda leaflets into the South. On Monday, the North's state media reported it was preparing to send around 12 million leaflets via some 3,000 balloons.