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

As outgunned but determined Ukrainian forces slowed the Russian advance and sanctions crippled the Russian economy, the military confirmed that its nuclear forces were on high alert, following President Putin’s order. While that raised the unimaginable specter of nuclear conflict, it was unclear what practical effect it had.

A tense calm reigned Monday in Kiev, where people lined up to buy food and water after two nights trapped inside by curfew. Explosions and gunfire were heard in embattled cities in eastern Ukraine, and 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 Ukrainian city of Mariupol. Around her, parents sought to console children and keep them warm.

Exact death tolls are unclear, but the U.N. human rights chief said 102 civilians have been killed and hundreds wounded in five days of fighting — warning that figure was likely a vast undercount — and Ukraine’s president said at least 16 children were among the dead. More than 500,000 people have fled the country  since the invasion, another U.N. official said Monday — among millions who have left their homes.

Still, a tiny 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.

President Zelensky’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 Mr. Putin’s adviser on culture — an unlikely envoy for ending the war and a sign of how Moscow views the talks. 

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 Mr. Putin’s aggression, the measures also risked pushing an increasingly cornered Mr. 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 Mr. 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 increasingly erratic Russian leader made a clear link between ever-tightening sanctions and his decision Sunday to raise Russia’s nuclear posture. He also pointed at “aggressive statements” by NATO, a reference to his long-held stance that the U.S.-led alliance is an existential threat to Russia.

On Monday, the Defense Ministry said extra personnel were deployed to Russian nuclear forces, and that the high alert status applies to all their components: the forces that oversee land-based intercontinental ballistic missiles, submarine-launched intercontinental ballistic missiles, and the fleet of nuclear-capable strategic bombers.

It was not immediately clear whether the Kremlin announcement means any nuclear-armed aircraft could already be in the air around Ukraine. But the move is a clear escalation.

While Russia and the United States typically have land- and submarine-based nuclear forces that are prepared for combat at all times, nuclear-capable bombers and other aircraft are not. 

U.S. and British officials played down Mr. Putin’s nuclear threat as posturing. But for many, they stirred up memories of the Cuban Missile Crisis and concerns that the West could be drawn into a direct conflict with Russia.

Mr. Putin also stepped up his rhetoric Monday, denouncing the U.S. and its allies as an “empire of lies.” He described Western allies as U.S. “satellites which humbly fawn on it, kowtow to it, copy its conduct and joyfully accept the rules.”

In another possible escalation, neighboring Belarus could send troops to help Russia as soon as Monday, according to a senior American intelligence official with direct knowledge of current U.S. intelligence assessments. The official spoke on condition of anonymity because he was not authorized to speak publicly.

U.S. officials say they believe the invasion has been more difficult, and slower, than the Kremlin envisioned, though that could change as Moscow adapts. The British Defense Ministry said Monday that the bulk of Putin’s forces are about 20 miles north of Kiev, their advance having been slowed by Ukrainian forces.

Battles also broke out in Ukraine’s second-largest city, Kharkiv, and strategic ports in the country’s south came under assault from Russian forces. Mariupol, a strategic port city on the Sea of Azov, is “hanging on,” an adviser to Mr. Zelensky, Oleksiy Arestovich, said. An oil depot was reportedly bombed in the eastern city of Sumy. Ukrainian protesters demonstrated against encroaching Russian troops in the port of Berdyansk.

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. Russians withdrew savings and sought to shed rubles for dollars and euros, while Russian businesses scrambled to protect their finances.

The U.S., European Union, and Britain also agreed to block selected Russian banks from the SWIFT system, which facilitates moving money around thousands of banks and other financial institutions worldwide. 

In New York, the 193-member U.N. General Assembly scheduled an emergency session Monday  on Russia’s invasion.

With the Ukrainian capital besieged, the Russian military offered to allow residents to leave Kiev via a safe corridor. The mayor of the city of nearly 3 million had earlier expressed doubt that civilians could be evacuated.

Ukrainian authorities have been handing out weapons to anyone willing to defend the city. Ukraine is also releasing prisoners with military experience who want to fight, and training people to make firebombs.