KYIV, Ukraine (AP) — Russia is preparing to step up its attacks on Ukraine using Iranian-made exploding drones, according to Ukraine’s president, as Moscow looks for ways to keep up the pressure on Kyiv after at least 63 Russian soldiers were killed in an attack in the latest battlefield setback for the Kremlin’s war strategy.

“We have information that Russia is planning a prolonged attack by Shaheds (exploding drones),” President Volodymyr Zelenskyy said in his nightly video address late Monday.

He said the goal is to break Ukraine’s resistance by “exhausting our people, (our) air defense, our energy,” more than 10 months after Russia invaded its neighbor.

Russian President Vladimir Putin appears to be exploring ways to regain momentum in his flawed war effort, which in recent months has been undermined by a Ukrainian counteroffensive backed by Western-supplied weapons. That has brought sharp rebukes in some Russian circles of the military’s performance.

In the latest embarrassment for the Kremlin, Ukrainian forces fired rockets at a facility in the eastern Donetsk region where Russian soldiers were stationed, killing 63 of them, according to Russia’s Defense Ministry. Other, unconfirmed reports put the death toll much higher.

It was one of the deadliest attacks on the Kremlin’s forces since the war began more than 10 months ago.

In the attack, Ukrainian forces fired six rockets from a HIMARS launch system and two of them were shot down, a Russian Defense Ministry statement said.

However, the Strategic Communications Directorate of Ukraine’s Armed Forces claimed Sunday that around 400 mobilized Russian soldiers were killed in a vocational school building in Makiivka and about 300 more were wounded. That claim couldn’t be independently verified. The Russian statement said the strike occurred “in the area of Makiivka” and didn’t mention the vocational school.

Satellite photos analyzed by The Associated Press show the apparent aftermath of the strike. An image from Dec. 20 showed the building standing. An image from Jan. 2 showed the building reduced to rubble. Other days had intense cloud cover, making seeing the site by standard satellite imagery impossible.

For the Russian military, the Iranian-made exploding drones are a cheap weapon which also spreads fear among troops and civilians. The United States and its allies have sparred with Iran over Tehran’s role in allegedly supplying Moscow with the drones.

The Institute for the Study of War said that Putin is looking to strengthen support for his strategy among key voices in Russia.

“Russia’s air and missile campaign against Ukraine is likely not generating the Kremlin’s desired information effects among Russia’s nationalists,” the think tank said late Monday.

“Such profound military failures will continue to complicate Putin’s efforts to appease the Russian pro-war community and retain the dominant narrative in the domestic information space,” it added.

Zelenskyy warned that in the coming weeks, “The nights may be quite restless.”

He added that during the first two days of the new year, which were marked by relentless nighttime drone attacks on Ukrainian cities and energy infrastructure, the country’s forces shot down more than 80 Iranian-made drones.

As well as hoping to wear down resistance to Russia’s invasion, the long-range bombardments have targeted the power grid to leave civilians at the mercy of biting winter weather as power outages ripple across the country.

“Every downed drone, every downed missile, every day with electricity for our people and minimal shutdown schedules are exactly such victories,” Zelenskyy said.

In the latest fighting, a Russian missile strike overnight on the city of Druzhkivka in the partially occupied eastern Donetsk region wounded two people, the deputy head of Ukraine’s presidential office, Kyrylo Tymoshenko, reported Tuesday.

Officials said the attack ruined an ice hockey arena described as the largest hockey and figure skating school in Ukraine.

Overnight Russian shelling was also reported in the northeastern Kharkiv region and the southeastern Dnipropetrovsk region.

In the recently retaken areas of the southern Kherson region, Russian shelling on Monday killed two people and wounded nine others, Kherson’s Ukrainian governor, Yaroslav Yanushevich, said Tuesday. He said the Russian forces fired at the city of Kherson 32 times on Monday.