KYIV, Ukraine (AP) — Russia pressed its invasion of Ukraine to the outskirts of the capital Friday after unleashing airstrikes on cities and military bases and sending in troops and tanks from three sides in an attack that could rewrite the global post-Cold War security order.

Explosions sounded before dawn in Kyiv as Western leaders scheduled an emergency meeting and Ukraine’s president pleaded for international help. The nature of the explosions was not immediately clear, but the blasts came amid signs that the capital and largest Ukrainian city was increasingly threatened following a day of fighting that left more than 100 Ukrainians dead.

President Zelenskyy said the government had information that “subversive groups” were encroaching on the city, and the U.S. secretary of state, Antony Blinken, said Kyiv “could well be under siege” in what U.S. officials believe is a brazen attempt by President Putin to dismantle the government and replace it with his own regime.

The U.S. defense secretary, Lloyd Austin, told lawmakers on a phone call Thursday evening that Russian mechanized forces that entered from Belarus were about 20 miles from Kyiv, according to a person familiar with the call.

The assault, anticipated for weeks by the U.S. and Western allies and undertaken by Mr. Putin in the face of international condemnation and cascading sanctions, amounts to the largest ground war in Europe since World War II.

Russian missiles bombarded cities and military bases in the first day of the attack, and Ukraine officials said they had lost control of the decommissioned Chernobyl nuclear power plant, scene of the world’s worst nuclear disaster. Civilians piled into trains and cars to flee and patrons of a hotel were directed into a shelter as explosions sounded in Kyiv.

“Russia has embarked on a path of evil, but Ukraine is defending itself and won’t give up its freedom,” Mr. Zelenskyy tweeted. His grasp on power increasingly tenuous, he called Thursday for even more severe sanctions than the ones imposed by Western allies and ordered a full military mobilization that would last 90 days.

Mr. Zelenskyy said in a video address that 137 “heroes,” including 10 military officers, had been killed and 316 people wounded. The dead included border guards on the Zmiinyi Island in the Odessa region, which was taken over by Russians.

He concluded an emotional speech by saying that “the fate of the country depends fully on our army, security forces, all of our defenders.” He also said the country had heard from Moscow that “they want to talk about Ukraine’s neutral status.”

President Biden was to meet Friday morning with fellow leaders of NATO governments in what the White House described as an “extraordinary virtual summit” to discuss Ukraine.

Mr. Biden announced new sanctions against Russia, saying Mr. Putin “chose this war” and had exhibited a “sinister” view of the world in which nations take what they want by force. Other nations also announced sanctions, or said they would shortly.

“It was always about naked aggression, about Putin’s desire for empire by any means necessary — by bullying Russia’s neighbors through coercion and corruption, by changing borders by force, and, ultimately, by choosing a war without a cause,” Mr. Biden said.

Mr. Blinken said in television interviews that he was convinced that Russia was intent on overthrowing the Ukrainian government, telling CBS that Mr. Putin wants to “reconstitute the Soviet empire” and that Kyiv was already “under threat, and it could well be under siege.” 

Fearing a Russian attack on the capital city, thousands of people went deep underground as night fell, jamming Kyiv’s subway stations.

At times it felt almost cheerful. Families ate dinner. Children played. Adults chatted. People brought sleeping bags or dogs or crossword puzzles — anything to alleviate the waiting and the long night ahead.

But the exhaustion was clear on many faces. And the worries.

“Nobody believed that this war would start and that they would take Kyiv directly,” said Anton Mironov, waiting out the night in one of the old Soviet metro stations. “I feel mostly fatigue. None of it feels real.”

The invasion began early Thursday with a series of missile strikes, many on key government and military installations, quickly followed by a three-pronged ground assault. Ukrainian and U.S. officials said Russian forces were attacking from the east toward Kharkiv, Ukraine’s second-largest city; from the southern region of Crimea, which Russia annexed in 2014; and from Belarus to the north.

Mr. Zelenskyy, who had earlier cut diplomatic ties with Moscow and declared martial law, appealed to global leaders, saying that “if you don’t help us now, if you fail to offer a powerful assistance to Ukraine, tomorrow the war will knock on your door.”

Although Mr. Biden said he had no plans to speak with Mr. Putin, the Russian leader did have what the Kremlin described as a “serious and frank exchange” with the French president, Emmanuel Macron.

Both sides claimed to have destroyed some of the other’s aircraft and military hardware, though little of that could be confirmed.

Hours after the invasion began, Russian forces seized control of the now-unused Chernobyl plant and its surrounding exclusion zone after a fierce battle, a presidential adviser, Myhailo Podolyak, told the Associated Press.

The Vienna-based International Atomic Energy Agency said it was told by Ukraine of the takeover, adding that there had been “no casualties or destruction at the industrial site.”