SEOUL—North Korean leader Kim Jong Un has exiled, imprisoned or executed suspected opponents of his diplomatic outreach to the U.S. and South Korea, while also targeting his country’s moneyed elite with asset seizures, according to a new report that details a purge of some 50 to 70 individuals.

The crackdown, portrayed as an anticorruption campaign in state-run media, suggests Mr. Kim is looking to silence critics and shore up his regime’s finances in the face of international sanctions, said U.S. security analysts and former South Korean intelligence officials.

Economic sanctions have pinched Pyongyang’s traditional sources of foreign currency, from exports to its access to the global banking system, and the confiscations represent a way for the regime to replenish much-needed funds.

The purge takes aim at officials who have used their powerful positions to amass wealth illicitly—albeit on a North Korean scale, according to analysts and the report from the North Korea Strategy Center, a Seoul-based think tank founded by a North Korean defector. The report’s findings are based on interviews with 20 current and former high-ranking members of the Kim regime.

In a widely watched Jan. 1 speech, Mr. Kim publicly declared a war against corruption—a rare statement by any North Korean leader, according to former South Korean intelligence officials.