A U.S. military intelligence agency believes that North Korea has learned how to mount nuclear bombs on ballistic missiles, a U.S. congressman said Thursday, citing a classified report.

But the Defense Intelligence Agency (DIA) still questions whether the secretive nation has mastered the sophisticated technology, according to Rep. Doug Lamborn (R-CO).

In a congressional hearing, he quoted the report as saying, "DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles; however the reliability will be low."

Both Secretary of Defense Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, refused to discuss the issue in the hearing, saying it is pertinent to a classified document.

The U.S. and its allies have paid keen attention to whether the North has succeeded in miniaturizing a nuclear bomb to be launched by missile.

The DIA's evaluation, if confirmed, likely would further raise public concerns about North Korea's threats. The North has conducted three nuclear tests and carried out several ballistic missile launches.

Meanwhile, Dempsey made clear that the military does not support the idea of redeploying tactical nuclear weapons to South Korea.

"We do not advocate the return of tactical nuclear weapons to the peninsula," he said.

Some conservative South Korean lawmakers have called for the U.S. to bring back those weapons to counter the North's threats.

Under the 2013 Defense Authorization Act, the Pentagon is working on a report on the feasibility and effectiveness of sending tactical nuclear weapons to Korea.

The U.S. pulled all of its tactical nuclear weapons, which can be delivered by artillery or missile, out of South Korea in 1991 as part of President George H.W. Bush's Presidential Nuclear Initiative. (Yonhap News)