North Korea is not believed to have mastered the technology to fit a nuclear warhead onto a long-range missile yet, U.S. Air Force Chief of Staff Gen. Mark Welsh said.

Welsh made the remark during a Pentagon briefing Monday, saying the commander of U.S. Northern Command "spends a lot of time worried about how we can be sure to take it out if they ever did develop the capability to combine a long-range missile with a warhead that was operable."

Asked if the North has reached the stage, Welsh said, "I don't think they're at that stage yet."

The North conducted its fourth nuclear test in January and a long-range missile test last month, demonstrating it is making strides in efforts to develop a long-range missile capable of reaching the U.S.

But many experts say the North has yet to master the technology to make nuclear warheads small enough to fit a long-range missile, and to make a missile capable of withstanding reentry into Earth's atmosphere.

Pentagon press secretary Peter Cook also said that the U.S. assessment has not changed.

"The United States has not seen North Korea demonstrate a capability to miniaturize a warhead, he said. "With regard to the ballistic missile threat, we still feel confident that we can deter and respond to a missile threat from North Korea," he said.

Cook also stressed that the U.S. is working closely with its allies and partners in the region to respond to the North's threats and to be prepared for "any eventuality." The ongoing joint exercises with South Korea are part of such efforts, he said.

"We've continued to maintain our posture in the region and remain confident in our capabilities to be able to respond to the North Korea threat," he said.

He also said the U.S. is confident it can "successfully knock it out of the sky" if the North launches a missile. (Yonhap)