The Pentagon spokesman added that "Mattis underscored the United States' ironclad commitment to defend Japan and provide extended deterrence using the full range of U.S. capabilities."

Pentagon Spokesman Dana White said late Wednesday in a statement that Defense Secretary James Mattis spoke by phone with his Japanese counterpart to discuss Pyongyang's ICBM test-firing and "both agreed that this test represents an escalation and unacceptable provocation that undermines regional security and stability."

North Korea launched the test missile from the west coast of the country and it fell into the Sea of Japan after traveling about 900 km (or nearly 560 miles), according to Japan's Defense Ministry.

State-run North Korean media boasted Wednesday its ICBM Hwasong-14 rocket launched this week was "capable of hitting any part of the world, along with nuclear weapons."

At the same time, there's now the possibility that the North's new ICBM could reach Alaska and some experts believe Pyongyang may also have the technology to deploy a warhead to California and eastward.

Out of the 17 tests of the U.S. ground-based midcourse defense (GMD) system since 1999, slightly more than half of them have been successful. That said, a test conducted May 30 of a simulated ICBM aimed at the West Coast destroyed the mock warhead, but no more tests were planned until late next year.

"All of this is creating a panic, if you will, because the ground-based interceptors in Alaska and California , while they have been tested, U.S. government agencies have critiqued the test as not being realistic," said John Park, director of the Korea Working Group at the Harvard Kennedy School. "There is concern about gap in capabilities there as well as with respect to defense of the homeland."

North Korea 's test-firing of an intercontinental ballistic missile this week came as the U.S. still had reliability issues with its homeland missile defense system and no guarantee it could destroy any incoming nuclear warhead from the rogue regime.

The intercontinental ballistic missile Hwasong-14 is seen in this undated photo released by North Korea's Korean Central News Agency in Pyongyang, July, 4 2017.

"Strategically, we've long known it was going to happen," said Bruce Klingner, a former CIA deputy division chief for the Korean peninsula and national security specialist at the Heritage Foundation, a conservative Washington think-tank. "North Korea said at the beginning of the year they will test [an ICBM] by the end of the year."

That's a bit slower than the CIA's late-1990s prediction that North Korea would have an ICBM by 2015, he said.

Klingner said debris recovered by the South Korean navy from earlier North Korean launches, including a three-stage rocket which launched a satellite in February 2016, showed it had a potential range of at least 10,000 km (about 6,200 miles).

Similarly, a 2012 rocket was later estimated to have a potential range of 12,000 km (or almost 7,500 miles), according to reports from South Korean media.

"One could probably reach all the way down to Miami," said Klingner.

"The current level of the North's nuclear and missile programs is a problem, but what is more troubling is their development is proceeding much faster than expected," South Korean President Moon Jae-in was quoted as saying Wednesday, according to Seoul-based state news agency Yonhap.

It said Moon's spokesman indicated the leader made those remarks in conversations held at the G-20 summit in Berlin.

Indeed, analysts pointed out North Korea has been picking up the pace of its ballistic missile tests, conducting at least 10 tests already this year. And confirmation that the secretive communist state successfully launched its first ICBM on Tuesday only added to the urgency of U.S. efforts to reduce the threat with a reliable homeland missile defense system.

Park, the director of the Korea Working Group at the Harvard Kennedy School, estimated that North Korea already had as many as 20 nuclear warheads, noting that while the ICBM was likely still in the development stage, Pyongyang "certainly can develop an assembly line to crank out more of those."

According to NBC News, President Barack Obama warned then President-elect Donald Trump during their first meeting after the election that the North Koreans were working on a long-range missile that could reach the U.S. mainland.

Trump's first meeting at the White House with Obama took place Nov. 10, and on Jan. 2, 2017, he tweeted: "North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won't happen."

Donald J. Trump: North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won't happen.

He also started to pressure China with tweets, but backed off by April as he reached out to Chinese President Xi Jinping for help.

However, last month those efforts appeared to sour when Trump tweeted: "While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried."

Donald J. Trump: North Korea is looking for trouble. If China decides to help, that would be great. If not, we will solve the problem without them! U.S.A.

Donald J. Trump: While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!

Donald J. Trump: Trade between China and North Korea grew almost 40% in the first quarter. So much for China working with us - but we had to give it a try!

The NBC News report cited multiple sources as saying the height the missile reached — more than 1,700 miles — and its capability to reach even as far as Alaska was something that "took U.S. intelligence by surprise."

That was a concern for U.S. security.

"North Korea seems to be moving faster than we are," said Thomas Karako, a senior fellow with the International Security Program and director of the Missile Defense Project at the Center for Strategic and International Studies, a Washington-based think-tank.

Added Karako, "What we really need to be doing is to have increased reliability, capability and capacity improvements relative to where we are today."

Sen. Dan Sullivan (R-Alaska), a member of the House Armed Services Committee, was leading a bipartisan effort to increase spending on missile defense, including space-based sensors. Some of the missile defense upgrades he pushed for are in the fiscal 2018 National Defense Authorization Act passed last week by the House Armed Services Committee.

The U.S. plans currently call for a total of 44 GMD interceptors to be available by the end of 2017 in California and Alaska, up from 36 today. Sullivan's plan, though, proposed to significantly expand the number ground-based interceptors to up to 100 nationally and would consider new locations too, including the East Coast and Midwest.

Some have been critical of the U.S. missile defense efforts, particularly spending and delays in getting a more reliable system.

The Missile Defense Agency has received more than $120 billion since 2002 on the GMD anti-missile system and there were plans to spend an additional $37 billion through 2021 to enhance its capabilities.

Vice Admiral James Syring, director of the Missile Defense Agency, testified last month to a House Armed Services subcommittee that the U.S. has "been on a journey over the last at least five to six years to improve the reliability of the entire [GMD] system."

He noted that the system was put in the field "very rapidly" in the early 2000s and changes were made along the way.

Even with the success of the May 30 ground-based interceptor test, however, Syring conceded more work is needed on the system.

"We are not there yet," he said. "I would not say we are comfortably ahead of the threat. I would say we are addressing the threat."