KYIV, Ukraine (AP) — Russian and Ukrainian officials met for talks Monday amid high hopes but low expectations for any diplomatic breakthrough, after Moscow ran into unexpectedly stiff resistance when it unleashed the biggest land war in Europe  since World War II.

Outgunned Ukrainian forces managed to slow the Russian advance,  and Western sanctions began to squeeze the Russian economy, but the Kremlin again raised the specter of nuclear war, reporting that its land, air and sea nuclear forces were on high alert following President Vladimir Putin's weekend order.

Stepping up his rhetoric, Putin denounced the U.S. and its allies as an "empire of lies."

A tense calm reigned in Kyiv, where people lined up to buy food, water and pet food after two nights trapped inside by a strict curfew, but social media video from Ukraine's second-largest city, Kharkiv, showed residential areas being shelled, with apartment buildings shaken by repeated, powerful blasts. Authorities in Kharkiv said at least seven people had been killed and dozens injured. They warned that casualties could be far higher.

"They wanted to have a blitzkrieg, but it failed, so they act this way," said 83-year-old Valentin Petrovich, who described watching the shelling from his downtown apartment. He spoke on condition that his full name not be used, fearing for his security.

The Russian military has denied targeting residential areas despite abundant evidence of shelling of homes, schools and hospitals.

Across the country, terrified families huddled overnight in shelters, basements or corridors.

"I sit and pray for these negotiations to end successfully, so that they reach an agreement to end the slaughter, and so there is no more war," said Alexandra Mikhailova, weeping as she clutched her cat in a makeshift shelter in the strategic southeastern port city of Mariupol. Around her, parents sought to console children and keep them warm.

The U.N. human rights chief said at least 102 civilians have been killed and hundreds wounded in more than four days of fighting — warning that figure is probably a vast undercount — and Ukraine's president said at least 16 children were among the dead.

More than a half-million people have fled the country  since the invasion, another U.N. official said, with many of them going to Poland, Romania and Hungary. And millions have left their homes.

Still, a sliver of hope emerged as the first face-to-face talks between Ukrainian and Russian officials since the war began opened Monday. The delegations met at a long table with the blue-and-yellow Ukrainian flag on one side and the Russian tricolor on the other.

Ukrainian President Volodymyr Zelenskyy's office said it would demand an immediate cease-fire and withdrawal of Russian troops.

But while Ukraine sent its defense minister and other top officials, the Russian delegation was led by Putin's adviser on culture — an unlikely envoy for ending the war and perhaps a sign of how seriously Moscow views the talks.

Also, the 193-nation U.N. General Assembly opened its first emergency session in decades in order to deal with the Ukraine invasion, with Assembly President Abdulla Shahid calling for an immediate cease-fire, maximum restraint by all parties and "a full return to diplomacy and dialogue."

Putin is not completely isolated and has talked to a series of heads of state in recent days, including a 90-minute call with French President Emmanuel Macron, who called for a truce. He has also spoken to the leaders of Israel, Armenia and other nations.

Meanwhile, Russia's Central Bank scrambled to shore up  the tanking ruble, and the U.S. and European countries upped weapons shipments  to Ukraine. While they hope to curb Putin's aggression, the measures also risked pushing an increasingly cornered Putin  closer to the edge — and inflicted pain on ordinary Russians.

In Moscow, people lined up to withdraw cash as the sanctions threatened their livelihoods and savings.

It wasn't immediately clear what Putin is seeking in the talks, or from the war itself, though Western officials believe he wants to overthrow Ukraine's government and replace it with a regime of his own, reviving Moscow's Cold War-era influence.

The Russian leader made a clear link between ever-tightening sanctions and his decision Sunday to raise Russia's nuclear posture. He also cited "aggressive statements" from NATO.

Moscow's Defense Ministry said that extra personnel were deployed to Russian nuclear forces and that the high alert applies to nuclear-capable intercontinental ballistic missiles, submarines and long-range bombers.

A senior U.S. defense official, speaking Monday on condition of anonymity, said the United States had yet to see any appreciable change in Russia's nuclear posture.

It was not immediately clear whether any nuclear-armed aircraft were in the air around Ukraine.

U.S. and British officials have played down Putin's nuclear threat as posturing. But for many, the move stirred up memories of the 1962 Cuban Missile Crisis and fears that the West could be drawn into direct conflict with Russia.

In another potential escalation, neighboring Belarus could send troops to help Russia as soon as Monday, according to a senior American intelligence official with direct knowledge of U.S. intelligence assessments. The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

Western officials say they believe the invasion has been slower, at least so far, than the Kremlin envisioned. British authorities said the bulk of Putin's forces were about 30 kilometers (20 miles) north of Kyiv.

Messages aimed at those soldiers popped up Monday on billboards, bus stops and electronic traffic signs across Kyiv. Some used profanity to encourage Russians to leave. Others appealed to their humanity.

"Russian soldier — Stop! Remember your family. Go home with a clean conscience," one read.

In other fighting, strategic ports in the country's south came under assault from Russian forces. Mariupol, on the Sea of Azov, is "hanging on," said Zelenskyy adviser Oleksiy Arestovich. An oil depot was reported bombed in the eastern city of Sumy. Ukrainian protesters demonstrated against encroaching Russian troops in the port of Berdyansk.

With the Ukrainian capital of nearly 3 million besieged, the Russian military offered to allow residents to leave Kyiv via a safe corridor.

In a war being waged both on the ground and online, cyberattacks hit Ukrainian embassies around the world, and Russian media.

Western nations ramped up the pressure with a freeze on Russia's hard currency reserves, threatening to bring Russia's economy to its knees. The U.S., European Union and Britain also agreed to block selected Russian banks from the SWIFT system, which facilitates the moving of money around thousands of banks and other financial institutions worldwide.

In addition to sanctions, the U.S. and Germany announced they will send Stinger missiles and other military supplies to Ukraine. The European Union — founded to ensure peace on the continent after World War II — is supplying lethal aid for the first time, including anti-tank weapons.

EU defense ministers were to meet Monday to discuss how to get the weaponry into Ukraine. A trainload of Czech equipment arrived Sunday and another was en route Monday, though blocking such shipments will clearly be a key Russian priority.

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Isachenkov and Litvinova reported from Moscow. Ellen Knickmeyer, Eric Tucker, Robert Burns and Hope Yen in Washington; James LaPorta in Miami; Francesca Ebel, Josef Federman and Andrew Drake in Kyiv; Mstyslav Chernov and Nic Dumitrache in Mariupol, Ukraine; Lorne Cook in Brussels; and other AP journalists from around the world contributed to this report.

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Follow the AP's coverage of the Ukraine crisis at https://apnews.com/hub/russia-ukraine