Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty

Ukrainian President Volodymyr Zelenskyy is praising his country's air defense, saying it "successfully repelled" a barrage of Russian missiles fired at Kyiv and other targets early Thursday.

Zelenskyy said the Ukrainian Air Force shot down 54 missiles and 11 attack drones.

"Our warriors all over Ukraine distinguished themselves and I thank all our Air Commands: Center, South, East and West," Zelenskyy said.

He specifically cited the efforts of the 96th Kyiv, 160th Odesa and the 208th Kherson anti-aircraft missile brigades, saying their "results are the best today."

Zelenskyy said several Russian missiles evaded Ukraine's air defense and hit several infrastructure targets.

"Our power engineers and repair crews are doing everything to make Ukrainians feel the consequences of the terrorists' strike as little as possible," Zelenskyy said.

As of Thursday evening, he said there were power outages in most regions of Ukraine, including the capital city Kyiv as well as the Lviv, Odesa, Kherson, Vinnytsia and Zakarpattia regions.

"But this is nothing compared to what could have happened, if it was not for our heroic anti-aircraft troops and air defense," Zelenskyy said.

Belarus' defense ministry said its air defenses had downed a Ukrainian S-300 missile in a field on Thursday morning during one of Russia's largest missile attacks against Ukraine since the start of the war.

The military commissar of the Brest region, Oleg Konovalov, played down the incident in a video message posted on social media by the state-run BelTA news agency, saying local residents had "absolutely nothing to worry about."

"Unfortunately, these things happen," Konovalov said.

He compared the incident to one in November when an S-300 believed to have strayed after being fired by Ukrainian air defenses landed in NATO member country Poland, and initial fears of an escalation in the war were rapidly defused.

Konovalov said the Ukrainian missile was shot down by the air defense forces around 10 a.m. local time Thursday. Fragments of the downed missile were found near the village of Gorbakha in the Brest region.

-ABC News' William Gretsky

Moscow has been "cruelly" targeting Ukrainian civilians by launching attacks against utilities during the winter, the U.S. Embassy in Kyiv said on Thursday.

The rebuke came as Russia fired missiles at cities throughout the country on Thursday. The General Staff of the Ukrainian Army said 69 missiles were launched, fewer than the 100 missiles that officials had estimated earlier in the morning. Officials said 54 missiles were intercepted.

Two civilians were killed as a result of shelling in the Kharkiv area, according to the region's governor.

"The Kremlin continues its escalating campaign of missile attacks, cruelly wielding cold & dark against" Ukrainians, U.S. Embassy officials said on Twitter. "Families are again hunkering down as critical infrastructure & other targets across the country are attacked."

Air raid sirens started wailing before 6 a.m. local time across Ukraine, sending residents scrambling into underground shelters in several cities. Missiles landed in Kyiv, Kharkiv, Odesa, Lviv and Zhtomyr.

Ukraine's defense systems intercepted some missiles, including 16 that were shot down near Kyiv, the capital, Mayor Vitali Klitschko said. Two homes in Kyiv were damaged by falling debris and three people were injured, he said.

Ukraine's Ministry of Defense said Russia had been "saving one of the most massive missile attacks since the beginning of the full-scale invasion for the last days of the year."

"They dream that Ukrainians will celebrate the New Year in darkness and cold," officials said. "But they cannot defeat the Ukrainian people."

-ABC News' Britt Clennett and Joe Simonetti

Russian forces early on Thursday launched a missile strike on Ukraine.

Oleksiy Arestovych, an adviser to Ukrainian President Volodymyr Zelenskyy, first posted on Twitter that more than 100 rockets were fired in several waves. Some rockets were reportedly fired from carriers in the sea, while others were reportedly fired by at least a dozen fighter aircraft.

Another presidential adviser, Mykhailo Podolyak, claimed on Twitter that more than 120 missiles had been launched "by the 'evil Russian world' to destroy critical infrastructure & kill civilians en masse."

The General Staff of the Ukrainian Army later said 69 missiles were launched, fewer than the 100 missiles that officials had estimated earlier in the morning.

At least one loud explosion was heard in Kyiv, where air raid sirens were ringing for several hours on Thursday morning.

-ABC News' Edward Szekeres, Britt Clennett and Joe Simonetti

Editor's note: This post has been updated to reflect an update issued by the Ukrainian Army on the number of missiles launched.