(LONDON) -- Russia launched a "massive" drone and missile strike on Ukraine overnight into Tuesday morning, Ukrainian President Volodymyr Zelenskyy said in a post to Telegram, prompting power outages in several regions around the country, according to the Energy Ministry in Kyiv.
Ukraine's air force said Russia launched 635 drones and 38 missiles into the country overnight, of which 587 drones and 34 missiles were shot down or suppressed. The impacts of missiles and drones were reported across 21 locations, the air force said.
Monday's night's attack was the largest Russian combined strike since Dec. 6, according to Ukrainian air force data analyzed by ABC News, and the third largest of the war to date.
Zelenskyy said Russia was "primarily targeting our energy sector, civilian infrastructure and literally all aspects of daily life."
At least three people were killed -- one person in Kyiv, one in Khmelnytskyi and a 4-year-old child in Zhytomyr -- Zelenskyy said. At least 13 regions were targeted, he added. Local officials reported that at least 11 people were also injured across Ukraine.
The Energy Ministry said in a post to Telegram that "emergency power cuts have been introduced in a number of regions of Ukraine. As soon as the security situation allows, rescue workers and energy specialists will begin to repair the damage caused by the attack in order to restore power supply to the regions as quickly as possible."
"Emergency power cuts will be lifted once the situation in the power grid has stabilized," the ministry added.
Ukraine's Deputy Energy Minister Mykola Kolisnyk told ABC News in an interview, "The enemy's plan is social instability through total blackout."
"This is not a hybrid threat," Kolisnyk said. "This is a military threat."
"The enemy was very precise, targeting high-voltage substations and power lines to break electricity connections between regions," Kolisnyk added. "These were strategic strikes aimed at the backbone of Ukraine's electricity network."
"Energy has become a battlefield," Kolisnyk said, noting that 80 percent of the overnight attacks were "deliberately aimed at energy infrastructure."
Though Kolisnyk said the national energy system "remains controlled, united and operational," he added that major cities nationwide "depend on large power generation. When high-voltage lines are hit, there are no easy alternatives."
"If the enemy increases the intensity of attacks, international support must increase at the same pace, with equipment and repair capacity," Kolisnyk said.
Zelenskyy said the attack "sends an extremely clear signal about Russia's priorities" as representatives from the warring parties engage in the latest round of U.S.-sponsored shuttle diplomacy intended to secure a peace deal.
"The attack comes just before Christmas, when people want to be with their families, at home, in safety," he wrote. "The attack actually comes at the height of negotiations aimed at ending this war."
Russian President Vladimir Putin, Zelenskyy said, "simply cannot accept the need to stop killing."
"This means that the world is not putting enough pressure on Russia," he added. "We need to react now. We need to push Russia towards peace and guaranteed security." Zelenskyy also called for "air defense for Ukraine, funding for arms purchases, the supply of energy equipment" to help blunt Russian attacks.
Ukraine continued its own long-range strike campaign overnight, with the Russian Defense Ministry reporting the downing of at least 44 drones.
Airports in Volgograd, Grozny, Magas and Vladikavkaz were temporarily closed amid the attacks, according to Russia's federal air transport agency, Rosaviatsiya.
In the Stavropol region, Gov. Vladimir Vladimirov said an attempted drone attack targeted facilities in Budennovsk, causing a fire in an industrial zone.
In the Rostov region, Gov. Yury Slyusar said drone debris damaged a fence and set fire to a house under construction in the village of Grushevskaya, which was later extinguished.
Meanwhile, on the diplomatic front, Ukrainian and Russian negotiators returned home after meetings with U.S. officials in Miami over the weekend.
Kremlin spokesman Dmitry Peskov said in remarks published Tuesday that Russia is waiting for information from the U.S. on talks with Ukrainian and European officials, after which he said Moscow would assess whether any developments "match the spirit of Anchorage" -- referring to the summit between Putin and President Donald Trump in Alaska in August.
Russian negotiator Kirill Dmitriev said he had returned to Moscow after taking part in the Miami talks. Earlier, the Kremlin said he would brief Putin on his return.
Vice President JD Vance on Monday said recent talks had achieved a "breakthrough" in that "all the issues are actually out in the open."
Peskov, though, told the Rossiya-1 television channel on Tuesday that Russia does not know what Vance was referring to.
Zelenskyy, meanwhile, said in social media posts on Monday that he will be briefed by the Ukrainian negotiating team on Tuesday morning. "There are 20 points of the plan," he said. "Not everything is perfect so far, but this plan is in place."
Zelenskyy said there is now a framework of security guarantees with European nations and the U.S. The "bilateral" and "legally binding" deal with the U.S. will need to be reviewed by Congress, he added. "As of today, this all looks quite solid and dignified. For now, however, these are working drafts prepared by our military."
"This indicates that we are very close to a real outcome," Zelenskyy added, noting that work was ongoing on a separate draft agreement regarding Ukraine's economic recovery.
"There are certain things we are not prepared to accept," Zelenskyy said. "And there are things -- of that I am sure -- that the Russians are not prepared to accept either. The Americans are currently continuing negotiations with Russian representatives. They will hold talks, and then we will receive feedback from them."
ABC News' Natalya Kushnir, Lama Hasan, Dragana Jovanovic and Will Gretsky contributed to this report.