Debris covered a street and firefighters rushed to rescue people from an apartment block hit by a Russian missile early Friday in Kharkiv, just hours after a shift in U.S. policy that will allow Ukraine to defend against such attacks by hitting targets in Russia with American-provided weaponry.

The shift is narrow in scope, granting Ukraine permission to use American air defense systems, guided rockets and artillery to fire into Russia only along Ukraine’s northeastern border, near Kharkiv. Fighting has been raging in the area for the past three weeks after Russian troops poured over the border to open a new front in the war.

But hitting targets with American weapons inside Russia had been a red line drawn by the Biden administration because of worries about escalation before the cross-border fighting began near Kharkiv. Russia has been launching missiles and gathering forces in the safety of its own territory, out of range of Ukraine’s Soviet-era weaponry.

The assaults have prompted urgent appeals from Ukraine for the Biden administration to remove the shackles, framing the use of Western weapons as a purely defensive tactic. Indeed, in granting permission, U.S. officials said the weapons should only be used in self-defense in the border region.

Still, it was a significant reversal that Ukraine hopes will help it regain its footing in a war that Russia is now dominating, and was a historic moment for the U.S. as well: It appeared to be the first time an American president had allowed the limited use of American weapons to strike inside the borders of a nuclear-armed adversary.