WASHINGTON — As North Korea rattles its nuclear saber, threatening to bomb the U.S. at “any moment,” a nerve-jangling question hangs in the air: If Pyongyang did launch a nuclear-armed missile at an American city, could the Pentagon’s missile defenses overcome their spotty test record and shoot it down beyond U.S. shores?

America has never faced such a real-life crisis, and although officials say they are confident the defenses would work as advertised, the Pentagon acknowledges potential gaps that North Korea or others might be able to exploit, someday if not immediately.

One possible vulnerability involves a foe’s “countermeasures,” or decoys carried aboard long-range offensive missiles to fool a U.S. interceptor missile into hitting the wrong target.

The Pentagon has poured at least $84 billion into missile defense over the past decade and is planning to spend another $3.3 billion over the next five years for a single element of the system, known as Groundbased Midcourse Defense, or GMD.

Its key part is a network of interceptor missiles designed to launch from underground silos, fly into the path of an enemy missile as it arcs through space and smash into it, destroying it.

That system has failed three of its last four intercept tests. The only success in that series was the most recent test, in June 2014.

A congressional watchdog agency, the Government Accountability Office, said in February that the Pentagon “has not demonstrated through flight testing that it can defend the U.S. homeland against the current missile defense threat.”

Adm. William Gortney, America’s homeland defender as head of U.S. Northern Command, told Congress this month that the nation needs “more capable forces and broader options.” Key improvements are in the works, including a “long-range discrimination” radar for more effective tracking of incoming missiles.

The North Koreans’ harsh rhetoric, including a threat in late February to deal “fatal blows at the U.S. mainland any moment,” is linked in part to its anger at U.S.-South Korean military exercises, which the North sees as a rehearsal for an invasion.

No one is predicting a bolt-out-of-the-blue North Korean nuclear attack, but the threat looms larger as the Koreans seemingly stride closer to fielding a nuclear-armed missile that can reach U.S. territory.

Already this year they have claimed a successful H-bomb test, put a satellite into space orbit, and claimed a successful simulated test of the warhead re-entry knowhow needed for a missile strike on the United States.

On Thursday, the North claimed to have successfully tested a solid-fuel rocket engine which, if true, would mark a significant further technological advance. The use of solid fuel reduces launch preparation time and thus shortens warning time for U.S. defenses.

More appears to be in store. State-run media reported earlier this month that leader Kim Jong Un ordered preparations for a “nuclear warhead explosion test” soon and test-firings of “several kinds of ballistic rockets able to carry nuclear warheads.” With increasing regularity, the North Koreans are claiming major advances on the nuclear front that have caught Washington’s attention, even if they are exaggerated.

President Barack Obama, who plans to address 50 heads of state gathered in Washington this week for a nuclear security summit, has said North Korea’s pursuit of nuclear and missile programs “increasingly imperils the United States.”

Of particular worry is a long-range missile under development in North Korea that the U.S. calls the KN-08. The Pentagon’s most recent public report on North Korea says the KN- 08 has a range of more than 3,400 miles, putting it into the category of an intercontinental ballistic missile, or ICBM.

Gortney said the KN-08 has “profound implications,” especially if it is deployed as a road-mobile weapon, meaning it could be moved and launched from vehicles that make it less vulnerable to detection. Such mobility, he said, would enable the North Koreans to elude or confound traditional U.S. pre-launch warning systems.

Gortney says the North Koreans may have figured out how to make a nuclear warhead small enough to fit atop a KN-08 missile.

“While the KN-08 remains untested, modeling suggests it could deliver a nuclear (weapon) to much of the continental United States,” Gortney told a Senate panel on March 10.