WASHINGTON (Reuters) - North Korea appears able to mount a miniaturized nuclear warhead on a missile but is still struggling with missile re-entry technology necessary for longer range strikes, a senior U.S. military official said on Thursday.

“I think they could mate a warhead with a delivery device. They’re just not sure (about) re-entry,” said the official, speaking to reporters on condition of anonymity.

“They’re endeavoring to overcome that.”

North Korea has carried out repeated nuclear and missile tests this year in defiance of U.N. Security Council resolutions and sanctions and claims it has the capability to mount a nuclear warhead on a missile.

Asked whether North Korea could mate the warhead to the missile, the official said: “I think they can.”

North Korea’s advancing nuclear weapons program is one of the major national security challenges that awaiting President-elect Donald Trump when he takes office on Jan. 20.

Trump has urged Beijing to do more to rein in its neighbor and told Reuters in May he was willing to talk to North Korean leader Kim Jong Un to try to stop Pyongyang’s nuclear program, in what would be a major shift in U.S. policy toward the isolated nation.

There are about 28,500 U.S. troops based in South Korea helping to defend the country against nuclear-armed North Korea, which has remained in a technical state of war with the South since the 1950-53 Korean conflict.

The official said the U.S. military was always reviewing potential responses to threats from the North, acknowledging greater attention on the issue in recent months.

“We’re preparing for everything that might evolve based on (Kim’s) very, very provocative behavior,” the official said.

The U.N. Security Council, which includes China, unanimously voted to impose new, tougher sanctions on North Korea a week ago.

But neither sanctions, imposed by Washington since 1950, nor the so-called six-party talks with Pyongyang to suspend its nuclear program in return for diplomatic rewards and energy assistance, have stopped North Korea from testing nuclear devices.

Earlier this year, Admiral Bill Gortney, the officer responsible for defending U.S. air space, told a Senate panel it was “prudent” for him to assume North Korea had the ability to miniaturize a nuclear weapon and put it on an intercontinental ballistic missile that could target the United States. [nL1N16I1O7]

The U.S. missile defense system is in the process of expanding its missile interceptors to 44 from 30 by the end of 2017. Forty will be at Fort Greely, Alaska, and four at Vandenberg Air Force Base in California.

The interceptors are designed to destroy a missile in space midway through its flight.