US officials said the B-52 flights demonstrated “extended deterrence capabilities” amid rising tension on the peninsula.
The aircraft, which can also carry conventional weapons, flew on 8 March, with another mission set for Tuesday.
Regional tension remains high after the North’s third nuclear test last month.
This is not the first time that B-52 bombers have been used as part of regular military drills between the US and South Korea.
“We’re drawing attention to the fact that we have extended deterrence capabilities that we believe are important to demonstrate in the wake of recent North Korean rhetoric,” US Pentagon press secretary George Little said.
“As North Korea threatened to attack South Korea with nuclear weapons, the exercise involving B-52s is meaningful, as it shows US commitment to provide its nuclear umbrella on the Korean peninsula,” said Kim Min-seok, spokesman for South Korea’s Defence Ministry.