SEOUL, Dec. 20 (AFP) -- South Korea and the United States conducted combined air drills, involving a U.S. B-52 strategic bomber and F-22 stealth fighters, on Tuesday, in an effort to strengthen the credibility of America's "extended deterrence," Seoul's defense ministry said.

The drills took place in the South's air defense identification zone southwest of its southern island of Jeju amid tensions caused by the North's launch of two medium-range ballistic missiles on Sunday.

The drills also involved the South's F-35A stealth jets and F-15K fighters.

This week, the F-22 jets from Kadena Air Base in Okinawa, Japan, are scheduled to participate in a separate combined training program involving the South's F-35A jets to strengthen allied capabilities to respond to the North's nuclear and missile threats.

"The deployment of the B-52H and F-22 fighters this time is part of an effort to reinforce the credibility of the U.S.' extended deterrence," the ministry said in a press release.

Extended deterrence refers to the U.S.' commitment to using a full range of its military capabilities, including nuclear, to defend its ally.

The ministry made the remarks referring to an agreement from the allies' defense ministerial talks last month to increase the frequency and intensity in the deployment of U.S. strategic assets to the Korean Peninsula.

"The South and the U.S. will further solidify the combined defense posture of the South Korea-U.S. alliance regarding the North's nuclear and missile threats by continuing to strengthen the alliance's capabilities and posture, including the U.S.' extended deterrence," the ministry said.

Sunday's missile firing was conducted at the North's key western rocket launch site in Tongchang-ri, where the North claimed to have tested a "high-thrust solid-fuel" rocket motor last week to develop another "new-type strategic weapon system."

The motor test was seen as part of Pyongyang's push to develop a solid-fuel intercontinental ballistic missile, which takes less launch preparation time than a liquid-fuel one that requires fuel injection and other procedures.

The U.S. F-22 jets previously visited the South for air drills in 2018.