KYIV, Ukraine — Russian troops pressed toward Ukraine’s capital Saturday after a night of explosions and street fighting sent Kyiv residents seeking shelter underground. The country’s leader refused an American offer to evacuate, insisting he would stay.

“The fight is here,” President Volodymyr Zelenskyy said.

Ukrainian officials reported some success in fending off assaults. Skirmishes reported on the edge of the city suggested that small Russian units were trying to clear a path for approaching forces, which a British official said “are still some way off.”

Russia claims its assault on Ukraine is aimed only at military targets, but civilians have been killed and injured during Europe’s largest ground war since World War II.

A missile struck a high-rise apartment building in the city’s southwestern outskirts near one of Kyiv’s two passenger airports, Mayor Vitali Klitchsko said, leaving a jagged hole of ravaged apartments over several floors. A rescue worker said six civilians were injured.

The mayor extended a 10 p.m.-7 a.m. curfew he imposed two days ago to run from 5 p.m. until 8 a.m. as of Saturday.

“All civilians on the street during the curfew will be considered members of the enemy’s sabotage and reconnaissance groups,” Klitschko said.

The conflict has already driven hundreds of thousands of Ukrainians from their homes. U.N. officials said more than 120,000 Ukrainians have left the country for Poland, Moldova and other neighboring nations.

Saturday’s street clashes followed two days of massive air and missile strikes as Russian soldiers moved in from the north, east and south. The assault pummeled bridges, schools and residential neighborhoods, and resulted in hundreds of casualties.

It was unclear in the fog of war how much of Ukraine was still under Ukrainian control and how much Russian forces have seized. Russia’s defense ministry claimed the Russian military had taken full control of the southern city of Melitopol, about 22 miles inland from the Azov Sea coast, and said Russia-backed separatists had made significant gains in the eastern region of Donbas.

Ukrainian soldiers take position on a bridge inside Kyiv on Friday. Associated Press/Emilio Morenatti

Western officials believe Russian President Vladimir Putin is determined to overthrow Ukraine’s government and replace it with a regime of his own. The invasion represented Putin’s boldest effort yet to redraw the map of Europe and revive Moscow’s Cold War-era influence.

Zelenskyy offered renewed assurance Saturday that the country’s military would stand up to the Russian invasion. In a defiant video recorded on a downtown Kyiv street, he said he remained in the city and that claims the Ukrainian military would put down arms were false.

“We aren’t going to lay down weapons. We will protect the country,” the Ukrainian president said. “Our weapon is our truth, and our truth is that it’s our land, our country, our children. And we will defend all of that.”

In a second video later Saturday, Zelenskyy accused the Russian forces of hitting civilian areas and infrastructure.

The U.S. government urged Zelenskyy early Saturday to evacuate Kyiv but he turned down the offer, according to a senior American intelligence official with direct knowledge of the conversation. The official quoted the president as saying that he needed anti-tank ammunition but “not a ride.”

The official spoke on condition of anonymity because he was not authorized to speak to the media.

Ukrainian President Zelenskyy: "If you have combat experience in Europe, and do not want to look at the indecision of politicians, you can come to our country and join us in defending Europe." pic.twitter.com/SbJgsqL8AI

— The Recount (@therecount) February 25, 2022

Ukraine’s health minister reported Saturday that 198 people, including three children, have been killed and more than 1,000 others have been wounded since the Russian offensive started before dawn Thursday. It was unclear whether the figure included both military and civilian casualties.

Ukrainian officials say hundreds of Russians have been killed in the first days of fighting. Russian authorities released no casualty figures.

The U.N. estimates that up to 4 million could flee if the fighting escalates. Refugees arriving in the Hungarian border town of Zahony said men of fighting age were not being allowed to leave Ukraine.

“My son was not allowed to come. My heart is so sore, I’m shaking, I can’t calm down, they did not let him come,” said Vilma Sugar, 68.

Many Kyiv residents spent another night in basements, underground parking garages and subway stations, heeding the warnings of city officials.

“We’re all scared and worried. We don’t know what to do then, what’s going to happen in a few days,” said Lucy Vashaka, 20, a worker at a small Kyiv hotel.

President of Ukraine Volodymyr Zelenskyy, Prime-Minister Denys Shmyhal and other officials share video they are in Kyiv, stay with people. "We are here, defending our state, and so it will be," Zelenskyy said. pic.twitter.com/JeTsRimXaK

— Euromaidan Press (@EuromaidanPress) February 25, 2022

A British official, Armed Forces Minister James Heappey, said fighting in the capital was so far confined to “very isolated pockets of Russian special forces and paratroopers.”

The United States and other global powers moved to freeze the assets of Putin and his foreign minister Friday as part of tougher sanctions on Russia as the invasion reverberated through the world’s economy and energy supplies.

Russia remained unbowed, vetoing a U.N. Security Council resolution demanding that it stop attacking Ukraine and withdraw troops immediately. The 11-1 vote, with China, India and the United Arab Emirates abstaining, showed significant opposition to Russia’s invasion of its smaller, militarily weaker neighbor.

A senior Russian official on Saturday shrugged off the wide-ranging sanctions that the U.S., the European Union and other allies slapped on Russia as a reflection of Western “political impotence.”

Dmitry Medvedev, the deputy head of Russia’s Security Council, warned that Moscow could react to the sanctions by opting out of the last remaining nuclear arms pact, freezing Western assets and cutting diplomatic ties with nations in the West.

“There is no particular need in maintaining diplomatic relations,” Medvedev said. “We may look at each other in binoculars and gunsights.”

People gather in the Kyiv subway on Friday, using it as a bomb shelter. Associated Press//Efrem Lukatsky

NATO, meanwhile, decided Friday to send parts of the alliance’s response force to help protect member nations in the east for the first time. NATO did not say how many troops would be deployed but added that it would involve land, sea and air power.

The Biden administration said it was sending Ukraine up to $350 million in arms and other defensive supplies from U.S. Department of Defense stockpiles, with another $250 million in defensive support possible. With the latest move, the U.S. has provided about $1 billion in defensive support to Ukraine over the past year.

Putin has argued that the West left him with no other choice by refusing to negotiate Russia’s security demands. Zelenskyy offered Friday to negotiate a key one: that Ukraine declare itself neutral and abandon its ambition of joining NATO.

The Kremlin said it accepted Kyiv’s offer to hold talks, but it appeared to be an effort to squeeze concessions out of the embattled Zelenskyy instead of a gesture toward a diplomatic solution.

The body of a serviceman is coated in snow next to a destroyed Russian military rocket launcher vehicle on the outskirts of Kharkiv, Ukraine, on Friday. Associated Press/Vadim Ghirda

Elsewhere in Kyiv, the body of a dead soldier lay near an underpass. Fragments of a downed aircraft smoked amid the brick homes of a residential area. Black plastic was draped over body parts found beside them. People climbed out of bomb shelters, basements and subways to face another day of upheaval.

“We’re all scared and worried. We don’t know what to do then, what’s going to happen in a few days,” said Lucy Vashaka, 20, a worker at a small Kyiv hotel.

At the Pentagon, press secretary John Kirby said the U.S. believes the offensive, including its advance on Kiev, has gone more slowly than Moscow had planned, noting that Ukraine forces have been fighting back. But he also said the military campaign is in an early stage and circumstances can change rapidly.

The Biden administration said Friday that it would move to freeze the assets of Putin and Lavrov, following the European Union and Britain in directly sanctioning top Russian leadership.

Zakharova, the Russian Foreign Ministry spokeswoman, called the sanctions against Putin and Lavrov “an example and a demonstration of a total helplessness” of the West.

Isachenkov and Litvinova reported from Moscow. Francesca Ebel, Josef Federman and Andrew Drake in Kyiv; Angela Charlton in Paris; Geir Moulson and Frank Jordans in Berlin; Raf Casert and Lorne Cook in Brussels; Nic Dumitrache in Mariupol, Ukraine; Matt Sedensky in New York; Jennifer Peltz at the United Nations; James LaPorta in Boca Raton, Fla., and Robert Burns, Matthew Lee, Aamer Madhani, Eric Tucker, Nomaan Merchant, Ellen Knickmeyer, Zeke Miller, Chris Megerian and Darlene Superville in Washington contributed.