North Korea's road-mobile KN-08 intercontinental ballistic missile is believed to be capable of delivering a nuclear warhead to the continental U.S., the U.S. northern commander said Thursday, amid growing concern Pyongyang could soon conduct its first mobile missile test.

"I assess that he has the ability to miniaturize a nuclear weapon and range the homeland with that warhead," Adm. William Gortney said during a House Armed Services Committee hearing in response to a question about the KN-08 missile, referring to North Korean leader Kim Jong-un.

Though the intelligence community assesses the probability of the North fielding a successful road-mobile ICBM with a miniaturized nuclear device capable of reaching the U.S. as low, Gortney stressed that as commander responsible for homeland defense, he chooses to assess the North has the capability.

"I think it's the prudent course of action. It's what I think the American people would like me to base my readiness assessment and be prepared to engage it. So we are prepared to engage it today, 24 hours a day and 365 days out of the year," he said.

Gortney made a similar statement in Wednesday's Senate Armed Services Committee hearing, saying that even though the KN-08 remains untested, "modeling suggests it could deliver a nuclear payload to much of the Continental United States.

Concerns have grown in recent weeks that Pyongyang could undertake yet another provocation, such as a nuclear test and a long-range missile launch, to mark Friday's birthday of founding leader Kim Il-sung, grandfather of current leader Kim Jong-un.

South Korea's Defense Ministry said Wednesday that the North has deployed one or two Musudan intermediate-range ballistic missiles near the eastern port city of Wonsan for a possible launch, and a launch could come on the occasion of the late leader's birthday.

U.S. officials have been quoted as saying that the North could attempt to test the longer-range KN-08.

The Guam Homeland Security and Office of Civil Defense said in a statement that it is closely monitoring the possibility of a North Korean missile launch. But it added that "no definitive reports of an immediate threat to Guam or the Northern Marianas (have) surfaced at this time."

A launch of either Musudan or KN-08 would mark the first time that the North has tested a mobile ballistic missile. The communist nation has displayed the KN-08 and other mobile missiles in military parades in recent years, but has never test-launched them.

U.S. officials have voiced strong concerns about the North's mobile missiles, especially the KN-08, as they can be fired from mobile launchers and are harder to keep an eye on. The U.S. has steadily strengthened its missile defense system to guard against such threats.

In Thursday's hearing, Gortney also spoke about challenges associated with a mobile missile.

"They're mobile and they're very easy to conceal. Previously, when North Korea assembles a rocket, we have intel that we can detect through all forms of intel. When you get into a road-mobile target, it's very very difficult to be able to track, quickly set up and shoot," the commander said.

"Most of my career I dropped bombs for a living and mobile targets would always cause me pause, and that's exactly why this is a tough challenge for us," he said.

The North has advanced ballistic missile technologies, and succeeded in putting satellites into orbit aboard long-range rockets twice, first in 2012 and again in February this year.

Experts say long-range rockets and ICBMs are basically the same with differences only in payloads.

Last week, the North claimed that it successfully carried out a ground test of a powerful ICBM engine, with leader Kim saying that the test "provided a firm guarantee for mounting another form of nuclear attack upon the U.S. imperialists."

State Department spokesman John Kirby said the U.S. is monitoring the situation closely.

"We've seen these reports, we're watching it closely. I can't predict one way or the other. And this is a regime, as you well know, that's difficult to predict. They do tend to conduct these kinds of activities ... around significant dates on the calendar," Kirby said.

"These kinds of activities do nothing to improve the security situation on the peninsula and serve as stark reminders of how important it is for us to stay, and we will, stay committed to our alliance commitments to South Korea," he said. (Yonhap)