SEOUL, South Korea — North Korea’s leader, Kim Jong-un, is disciplining the leadership of his country’s most powerful military organization, the latest sign of his efforts to tighten his grip on party elites and the armed forces amid a nuclear standoff with the United States, South Korea’s main intelligence agency said on Monday.

Analysts and experts pay close attention to any signs of rumbling within the secretive regime in Pyongyang, seeking to determine possible implications for the stability of Mr. Kim’s rule and for his nuclear and missile programs. They have said that Mr. Kim appeared to be using his tactic of instilling fear in the elites in order to strengthen his control as the country braced for the pain that is likely to result from recently imposed United Nations sanctions.

During a closed-door parliamentary briefing on Monday, South Korea’s National Intelligence Service told lawmakers that the North’s General Political Bureau was being “audited” by the country’s leadership for the first time in 20 years. The military organization’s director, Vice Marshal Hwang Pyong-so, and his deputies were “punished,” according to lawmakers who briefed reporters after the session.

The General Political Bureau oversees the political life of North Korea’s 1.1-million-strong People’s Army, monitoring the loyalty of its officers. Vice Marshal Hwang has been widely considered the No. 2 man in Mr. Kim’s totalitarian regime.