Kim Yong-jin would be the highest-ranking official known to have been executed since 2013, when North Korea confirmed in a rare announcement that Kim Jong-un had executed his own uncle and No. 2 official, Jang Song-thaek, on charges of factionalism, corruption and plotting to overthrow his government.

The ministry official who spoke on the condition of anonymity said that Kim Yong-chol, the leader of the United Front Department, had spent a month at a re-education camp on suspicion of abuse of power and that he had been released in mid-August.

Kim Yong-chol is seen as a hard-liner by South Korean officials. He was accused of helping orchestrate recent armed provocations by the North along the inter-Korean border, including an artillery barrage against a South Korean island in 2010, when he was the army’s intelligence chief. The Unification Ministry official said Mr. Kim would now need to prove his loyalty, which the official said raised the possibility that the North could take more aggressive actions toward South Korea.

Since taking power in 2011, Kim Jong-un has frequently reshuffled the party and military elites as he has consolidated his authority in North Korea, which his family has ruled for seven decades. Mr. Kim has also executed dozens of top officials in what President Park Geun-hye of South Korea has called a “reign of terror,” according to South Korean intelligence officials.

It remains difficult to independently verify reports of executions and purges in the secretive North. North Korea rarely announces them.