North Korea is now able to make a miniaturized nuclear warhead that could be delivered by an intercontinental ballistic missile, U.S. intelligence officials told NBC and The Washington Post

The successful miniaturization of a nuclear weapon would mark a major milestone for North Koreas notorious nuclear program, further escalating international tensions over the countrys pursuit of nuclear-capable ICMBs and raising concerns that the secretive state is already able to reach the mainland United States with such a device.

U.S. President Donald Trump issued a stern response on Tuesday to North Korea after reports of their nuclear advancements surfaced, warning of serious consequences for the country if it escalates the crisis.

North Korea best not make anymore threats to the United States. They will be met with fire and fury like the world has never seen, Trump said.

North Korean leader Kim Jong Un claimed late last month that his government had the ability to strike the U.S. mainland, following the successful test of two ICBMs. But it remains uncertain exactly how advanced North Korean missile technology is, and whether it can accurately deliver a long-range missile to an intended target.

Its also not clear whether North Korea has successfully tested the miniaturized nuclear weapons, and HuffPost was not able to independently verify the contents of the confidential U.S. intelligence report.

The United States has long attempted to prevent the growth of North Koreas nuclear program and missile technology, using a combination of sanctions, diplomacy, cyber attacks and the threat of a military option. None have completely stemmed the tide of North Koreas advancements, and many analysts have been alarmed at how rapidly the country has progressed in recent years.

Kim Jong Un has tested more missiles than any of his predecessors since he took over control of the country from his father in 2011, and his string of missile tests and aggressive rhetoric have perpetuated an ongoing international debate over how to handle the crisis. On Saturday, the United Nations Security Council issued unanimous sanctions on North Korean exports and condemned the countrys recent missile tests.

China notably joined in on the sanctions, although it remains to be seen how tightly the country will enforce the measures. North Korea reacted angrily to the new round of sanctions, issuing a vague threat against the U.S. through its official news agency.

Trumps administration has been grappling with ways to address the issue, and has tried to put public pressure on China to exert its influence on North Korea to rein in its nuclear ambitions. Trump officials have also stated that the U.S. would not tolerate a situation in which North Korea had the power to attack American cities with nuclear ICBMs.

The United States has also been advancing the development of a Ground-based Midcourse Defense system, or GMD, that is intended to intercept long-range missiles headed toward the U.S. and Canada. But the system is far from foolproof, and some experts have criticized it for having a mediocre test record that cant guarantee absolute safety in the event of a nuclear attack.