SEOUL, South Korea — South Korea was investigating the possibility of a North Korean cyberattack after the computer networks of three broadcasters and three banks were paralyzed Wednesday, while the North’s young leader, Kim Jong Un, increased his threats against the South and the United States.

During North Korean military drills aimed at countering any “U.S. war moves,” the North’s official Korean Central News Agency said, Kim threatened to destroy South Korean military and government installations and U.S. military bases in the Pacific, along with “the relevant facilities of countries.”

South Korea’s two leading television stations, KBS and MBC, maintained normal broadcasts but said their computers were frozen. The cable channel YTN reported a similar problem. The KBS website was shut down.

Shinhan Bank, the country’s fourth-largest lender, reported that its Internet banking servers had been blocked temporarily. Technicians restored operations.

Two other banks, NongHyup and Jeju, reported that operations at some of their branches had been paralyzed after computers were infected with viruses and their files were erased, the commission said. After two hours, the banks’ operations returned to normal, they said. A fourth bank, Woori, reported a hacking attack, but said it had suffered no damage.

The government and military cautioned that it was still too early to point the finger at the North, which has been threatening “pre-emptive nuclear attacks” and other, unspecified actions against its southern neighbor for conducting military exercises with the United States this month and for supporting new U.S.-led U.N. sanctions against the North.