U.N. Secretary-General Ban Ki-moon said Monday North Korea’s reported execution of Jang Song-thaek, once a key player in its leadership, was "very dramatic and surprising," but he cautioned regional powers not to overreact to the new developments on the peninsula.

"The recent report coming from DPRK (North Korea) was very dramatic and surprising surrounding the execution of Jang Song Thaek, who used to be No. 2 leader," Ban said at a new conference.

It was the U.N. leader’s first formal response to Pyongyang’s announcement that Jang was executed immediately after a special military trial. The uncle of the North’s supreme ruler, Kim Jong-un, was accused of attempting to overthrow the regime and dealing a heavy blow to the communist nation’s economy through policy failures.

Ban, formerly a South Korean foreign minister, said North Korea’s move was basically a violation of human rights law.

He called for neighboring nations to deal with the situation in a calm manner.

"At this time, I would appeal to all the parties concerned, surrounding the Korean Peninsula, while they must be vigilantly and carefully watching the development of situation, not to take any premature actions," he said. "I do not hope that because of that there will be some increase of tensions on the Korean Peninsula."

Ban’ remarks came as U.N. observers speculate whether the purge and execution of Jang will affect the possibility of more sanctions on the North.

A panel of U.N. experts has reached a conclusion that the North clearly violated existing international sanctions in July, when its Chong Chon Gang cargo ship was interdicted by Panamanian authorities, according to a U.N. source. It was carrying old Cuban arms to North Korea.

"The experts’ panel under the U.N. Security Council’s North Korea sanctions committee plans to submit to the committee within this year, describing the shipment by the Chong Chon Gang ship as a violation of U.N. resolutions," the source told Yonhap News Agency, requesting anonymity.

China, a permanent member of the council, agreed to that conclusion, which will be included in a final "incident report" not to be open to public, added the source.

The source said the news of Jang’s fate has emerged as a key factor in considering expanding U.N. sanctions on North Korea for the shipment.

U.N. Security Council members tend to consider internal politics in expanding sanctions on a certain nation, said the source. (Yonhap News)