KYIV, Ukraine (AP) – The U.S. will allow Ukraine to use American-supplied longer-range weapons to conduct strikes deeper inside Russian territory, a long-sought request by Kyiv.

It is not yet clear if there are limits on Ukraine’s use of the Army Tactical Missile System, or ATACMS, as there have been on other U.S. missile systems. Their deployment could—at least initially—be limited to Russia’s Kursk region, where Ukrainian troops seized territory earlier this year.

Since the first year of the war, Ukrainian leaders have lobbied Western allies to allow them to use advanced weapons to strike key targets inside Russia—which they hope would erode Moscow’s capabilities before its troops reach the front line and could make it more difficult for the Russian forces to strike Ukrainian territory. It could also serve as a deterrent force in the event of future cease-fire negotiations.

The U.S. has long opposed the move, with President Joe Biden determined to avoid any escalation that he felt could draw the U.S. and other NATO members into direct conflict with nuclear-armed Russia. The Kremlin warned on Monday that the decision adds “fuel to the fire.”

The decision comes in the waning days of Mr. Biden’s presidency, before President-elect Donald Trump assumes office. Mr. Trump has said he would bring about a swift end to the war, which many fear could force unpalatable concessions from Kyiv.

What Are ATACMS?

The ballistic missiles, developed by U.S. aerospace and defense company Lockheed Martin, have nearly double the striking distance—up to 190 miles—of most of the weapons in Ukraine’s possession. They carry a larger payload and have more precise targeting for pinpoint attacks on air fields, ammunition stores and strategic infrastructure.

The United States has supplied Ukraine with dozens of ATACMS (pronounced attack-ems) and they have been used to destroy military targets in Russian-occupied parts of Ukraine such as Crimea—but not on Russian soil.

What Is Mr. Biden Allowing Ukraine to Do?

Mr. Biden authorized Ukraine to use the ATACMS to strike deeper inside Russia, according to a U.S. official and three other people familiar with the matter.

The longer-range missiles are likely to be used in response to North Korea’s decision to send troops to support Kremlin forces, according to one of the people familiar with the development. Pyongyang’s troops are apparently being deployed to help the Russian army drive Ukrainian forces out of Russia’s Kursk border region, where they launched an incursion in August.

The officials spoke on condition of anonymity because they were not authorized to discuss the U.S. decision publicly.

It was the second time that Washington has expanded Ukraine’s authority to use its U.S.-provided weapons systems inside Russian territory.

In May, after Russia’s offensive into the Kharkiv region threatened to stretch Ukrainian forces thin, Mr. Biden permitted the use of HIMARS systems—with a range of 50 miles—to quell that advance. That decision helped Ukrainian soldiers stabilize the fight for a time by forcing Russian forces to pull back military assets.

Why Does Ukraine Need Longer-range Weapons?

Ukraine has been asking its Western allies for longer-range weapons in order to alter the balance of power in a war where Russia is better resourced, and strike with precision air bases, supply depots and communication centers hundreds of miles over the border.

It hopes the weapons would help blunt Russia’s air power and weaken the supply lines it needs to launch daily strikes against Ukraine and to sustain its military ground offensive into Ukraine.

If used in Kursk, the weapons would likely require Russian forces preparing for counterattacks to push back valuable equipment and manpower and complicate battle plans.

In lieu of Western weapons, Ukraine has been regularly striking Russia with domestically produced weapons, with some capable of traveling up to 620 miles, but still lacks sufficient quantities to do serious long-term harm.

Will the Decision Change the Course of the War?

Ukrainian leaders are being cautious about the announcement—and senior U.S. defense and military leaders have persistently argued that it will not be a gamechanger. They also have noted that Russia has moved many key assets out of range.

“I don’t believe one capability is going to be decisive and I stand by that comment,” Defense Secretary Lloyd Austin has said, noting that the Ukrainians have other means to strike long-range targets.

Analysts have also suggested the effect could be limited.

“Today, many in the media are talking about the fact that we have received permission to take appropriate actions. But blows are not inflicted with words. Such things are not announced. The rockets will speak for themselves,” Ukrainian President Volodymyr Zelenskyy said of the announcement.

The effect of the decision depends on the rules set for the weapons’ use.

If strikes are allowed across all of Russia, they could significantly complicate Moscow’s ability to respond to battlefield demands.

If strikes are limited to the Kursk region, Russia could relocate its command centers and air units to nearby regions, blunting the effect of those logistical challenges. That would also mean many of the valuable targets Ukrainian officials have expressed desire to hit may still be beyond reach.

Either way, Pentagon spokesman Lt. Col. Charlie Dietz has noted the ATACMS would not be the answer to the main threat Ukraine faces from Russian-fired glide bombs, which are being fired from more than 180 miles away, beyond the ATACMS’ reach.

In addition, the overall supply of ATACMS is limited, so U.S. officials in the past have questioned whether they could give Ukraine enough to make a difference—though some proponents say that even a few strikes deeper inside Russia would force its military to change deployments and expend more of its resources.

Jennifer Kavanagh, director of military analysis at Defense Priorities, said the U.S. decision would not alter the course of the war.

“To really impose costs on Russia, Ukraine would need large stockpiles of ATACMS, which it doesn’t have and won’t receive because the United States’ own supplies are limited,” she said. “Moreover, the biggest obstacle Ukraine faces is a lack of trained and ready personnel, a challenge that neither the United States nor its European allies can solve and that all the weapons in the world won’t overcome.”

What Are the Key Remaining Questions?

In addition to it being unclear what, if any, restrictions the U.S. will impose on the weapons’ use, it is also not known how many the U.S. will give to Ukraine.

While the U.S. has provided ATACMS to Ukraine in various military aid packages, the Defense Department will not disclose how many have been sent or exactly how many of those missiles the Pentagon has. Estimates suggest the U.S. has a number that is in the low thousands.

The recent American election raises questions over how long this policy will be in place. Mr. Trump has repeatedly criticized the Biden administration’s spending to support Ukraine—and could reverse moves like this one.

On the other hand, it is also not clear whether other allies might step up: The decision may encourage Britain and France to allow Ukraine to use Storm Shadow missiles, also known as SCALP missiles, with a range of 155 miles.