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

It was not clear if there would be 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 — a move they hope would erode Moscow’s capabilities before their troops reach the eastern front line and could make it more difficult for them to strike Ukrainian territory. It could also serve as a deterrent force in the event of future cease-fire negotiations.

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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 Monday that the decision adds “fuel to the fire.”

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

What are ATACMS?

The ballistic missiles, developed by U.S. aerospace and defense company Lockheed Martin, have nearly double the striking distance — up to 300 kilometers (190 miles) — of most of the weapons in Ukraine’s possession. They can carry out pinpoint attacks on air fields, ammunition stores and strategic infrastructure.

The United States supplied Ukraine with ATACMS last year and they have been used to destroy military targets in Russian-occupied territories of Ukraine — but not on Russian soil.

What is Biden allowing Ukraine to do?

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 marks the second time that Washington has permitted Ukraine to use its weapons systems inside Russian territory.

In May, after Russia’s offensive into the Kharkiv region threatened to stretch Ukrainian forces thin, Biden permitted the use of HIMARS systems — with a range of 80 kilometers (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 the war where Russia is better resourced, and strike with precision air bases, supply depots and communication centers hundreds of kilometers (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 1,000 kilometers (620 miles), but still lacks sufficient quantities to do serious long-term harm.

Will the decision change the course of the war?

The effect 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.

Ukrainian leaders are being cautious about the announcement.

“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,” said Ukrainian President Volodymyr Zelenskyy of the announcement.

Pentagon spokesman Lt. Col. Charlie Dietz has noted the ATACMS wouldn’t be the answer to the main threat Ukraine faces from Russian-fired glide bombs, which are being fired from more than 300 kilometers (180 miles) away, beyond the ATACMS’ reach.

Ukrainian leaders have also said that Russian forces have had time to pull back assets beyond the range of the missiles in the time it has taken the U.S. to make the decision.

“The decision came far too late. Had it been made at the beginning of the fall, it might have disrupted Russia’s counteroffensive in the Kursk region. And if it had been made even earlier, it could have derailed the offensive in the Pokrovsk direction,” said Glib Voloskyi, an analyst at the CBA Initiatives Center, a think tank.

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

And the decision may also encourage Britain and France to allow Ukraine to use Storm Shadow missiles, also known as SCALP missiles, with a range of 250 kilometers (155 miles).

Associated Press writers Hanna Arhirova in Kyiv, Ukraine, Aamer Madhani in Manaus, Brazil, and Colleen Long, Zeke Miller, Matthew Lee and Ellen Knickmeyer in Washington contributed to this report.