KYIV, Ukraine — A Russian missile barrage on the Ukrainian power grid sent the war spilling over into neighboring countries Tuesday, hitting NATO member Poland and cutting electricity to much of Moldova.

It was Russia’s biggest barrage yet, and some of the missiles crossed into Poland, where two people were killed, according to an American official. It marked the first time in the war that Russian weapons have come down on a NATO country.

Russia pounded Ukraine’s energy facilities Tuesday with its biggest barrage of missiles yet, striking targets across the country and causing widespread blackouts. A senior American intelligence official said missiles crossed into NATO member Poland, where two people were killed.

The Russian Defense Ministry denied being behind “any strikes on targets near the Ukrainian-Polish border” and said in a statement that photos of purported damage “have nothing to do” with Russian weapons.

A NATO official, speaking on condition of anonymity, said the alliance was looking into reports of a strike in Poland. The U.S. National Security Council said it was also checking into the reports.

A Polish government spokesman, Piotr Mueller, did not immediately confirm the information from a senior American intelligence official, who spoke on condition of anonymity because of the sensitive nature of the situation.

A second person confirmed that apparent Russian missiles struck a site in Poland about 15 miles from the Ukrainian border.

Mr. Mueller said top leaders were holding an emergency meeting due to a “crisis situation.”

Polish media reported that two people died Tuesday afternoon after a projectile struck an area where grain was drying in Przewodów, a Polish village near the border with Ukraine.

Neighboring Moldova was also affected. It reported massive power outages after the strikes knocked out a key power line that supplies the small nation, an official said.

The strikes plunged much of Ukraine into darkness and drew defiance from President Zelensky, who shook his fist and declared: “We will survive everything.”

Mr. Zelensky said Russia fired at least 85 missiles, most of them aimed at the country’s power facilities, and blacked out many cities.

“We’re working, will restore everything. We will survive everything,” the president vowed. His energy minister said the attack was “the most massive” bombardment of power facilities in the nearly 9-month-old Russian invasion, striking both power generation and transmission systems.

The minister, Herman Haluschenko, described the missile strikes as “another attempt at terrorist revenge” after military and diplomatic setbacks for the Kremlin. He accused Russia of “trying to cause maximum damage to our energy system on the eve of winter.”

The aerial assault, which resulted in at least one death in a residential building in the capital, Kyiv, followed days of euphoria in Ukraine sparked by one of its biggest military successes — the retaking last week of the southern city of Kherson.

The power grid was already battered by previous attacks that destroyed an estimated 40 percent of the country’s energy infrastructure.

Russian President Vladimir Putin has not commented on the retreat from Kherson since his troops pulled out in the face of a Ukrainian offensive. But the stunning scale of Tuesday’s strikes spoke volumes and hinted at anger in the Kremlin.

By striking targets in the late afternoon, not long before dusk began to fall, the Russian military forced rescue workers to labor in the dark and gave repair crews scant time to assess the damage by daylight.

More than a dozen regions — among them Lviv in the west, Kharkiv in the northeast and others in between — reported strikes or efforts by their air defenses to shoot missiles down. 

At least a dozen regions reported power outages, affecting cities that together have millions of people. Almost half of the Kyiv region lost power, authorities said. Ukrainian Railways announced nationwide train delays.

Mr. Zelensky warned that more strikes were possible and urged people to stay safe and seek shelter.

“Most of the hits were recorded in the center and in the north of the country. In the capital, the situation is very difficult,” said a senior official, Kyrylo Tymoshenko.

He said a total of 15 energy targets were damaged and claimed that 70 missiles were shot down. A Ukrainian Air Force spokesman said Russia used X-101 and X-555 cruise missiles.

As city after city reported attacks, Mr. Tymoshenko urged Ukrainians to “hang in there.”

With its battlefield losses mounting, Russia has increasingly resorted to targeting Ukraine’s power grid, seemingly hoping to turn the approach of winter into a weapon by leaving people in the cold and dark.