KYIV, Ukraine (AP) — Ukrainian and Russian officials are due to hold talks at a venue on the Belarusian border with Ukraine, President Volodymyr Zelenskyy’s office said on Sunday.

Zelenskyy’s office said on the Telegram messaging app that the two sides would meet at an unspecified location on the Belarusian border. Russia had proposed holding talks in Gomel, a Belarusian city near the border with Ukraine.

Ukraine’s Deputy Interior Minister Evgeny Yenin said the talks between Russian and Ukrainian delegations will take place Monday morning.

The possibility of the meeting was in jeopardy as Zelenskyy insisted the meeting be held in a “neutral” place and not a location that was used as a staging ground for Russian troops.

It is not clear what the talks will entail, but they have been framed as potential negotiations to end the Russian aggression in the region.

Meanwhile, in a dramatic escalation of East-West tensions over Russia’s invasion of Ukraine, President Vladimir Putin ordered Russian nuclear forces put on high alert Sunday in response to what he called “aggressive statements” by leading NATO powers.

The directive to put Russia’s nuclear weapons in an increased state of readiness for launch raised fears that the crisis could boil over into nuclear warfare, whether by design or mistake.

Putin’s step is “potentially putting in play forces that, if there’s a miscalculation, could make things much, much more dangerous,” said a senior U.S. defense official, speaking on condition of anonymity.

Amid the mounting tensions, the Kremlin’s ultimate aims in Ukraine — and what steps might be enough to satisfy Moscow — remained unclear.

The fast-moving developments came as Russian troops drew closer to Kyiv, a city of almost 3 million, street fighting broke out in Ukraine’s second-largest city, Kharkiv, and strategic ports in the country’s south came under pressure from attackers. Ukrainian defenders put up stiff resistance that appeared to slow the invasion.

“I wish I had never lived to see this,” said Faina Bystritska, 87, a Jewish survivor of World War II. She said sirens blare almost constantly in her hometown, Chernihiv, which is about 90 miles from Kyiv and under attack. Residents have been told not to switch on any lights “so we don’t draw their attention.” She has been living in a hallway, away from windows, the better to protect herself.

“The window glass constantly shakes, and there is this constant thundering noise,” she said.

The top official in the European Union outlined plans by the 27-nation bloc to close its airspace to Russian airlines and fund the purchase of weapons for Ukraine.

“For the first time ever, the European Union will finance the purchase and delivery of weapons and other equipment to a country that is under attack,” said European Commission President Ursula von der Leyen. The EU will also ban some pro-Kremlin media outlets, she said.

Putin, in giving the nuclear alert directive, cited not only statements by NATO members — who have rushed to reinforce the military alliance’s members in Eastern Europe — but the hard-hitting financial sanctions imposed by the West against Russia, including Putin himself.

Speaking at a meeting with his top officials, Putin told his defense minister and the chief of the military’s General Staff to put the nuclear deterrent forces in a “special regime of combat duty.”

“Western countries aren’t only taking unfriendly actions against our country in the economic sphere, but top officials from leading NATO members made aggressive statements regarding our country,” Putin said in televised comments.

U.S. defense officials would not disclose their current nuclear alert level except to say that the military is prepared all times to defend its homeland and allies.

White House press secretary Jen Psaki told ABC that Putin is resorting to a pattern he used in the weeks before the invasion, “which is to manufacture threats that don’t exist in order to justify further aggression.”

Psaki added the invasion in Ukraine will also be included in U.S. President Joe Biden’s State of the Union address on Tuesday.

Earlier Sunday, the Ukrainian capital, Kyiv, was eerily quiet after huge explosions lit up the morning sky and authorities reported blasts at one of the airports. Only an occasional car appeared on a deserted main boulevard as a strict 39-hour curfew kept people off the streets. Terrified residents instead hunkered down in homes, underground garages and subway stations in anticipation of a full-scale Russian assault.

“The past night was tough – more shelling, more bombing of residential areas and civilian infrastructure,” Zelenskyy said.

Ukraine’s health ministry said on Sunday that 352 civilians, including 14 children, had been killed since the beginning of Russia’s invasion of Ukraine.

It also said that 1,684 people, including 116 children, had been wounded.

Russia has not released any casualty information.

Until Sunday, Russia’s troops had remained on the outskirts of Kharkiv, a city of 1.4 million about 12.4 miles south of the border with Russia, while other forces rolled past to press the offensive deeper into Ukraine.

Videos posted on Ukrainian media and social networks showed Russian vehicles moving across Kharkiv and Russian troops roaming the city in small groups. One showed Ukrainian troops firing at the Russians and damaged Russian light utility vehicles abandoned nearby.

The images underscored the determined resistance Russian troops face while attempting to enter Ukraine’s bigger cities. Ukrainians have volunteered en masse to help defend the capital, Kyiv, and other cities, taking guns distributed by authorities and preparing firebombs to fight Russian forces.

Ukraine’s government also is releasing prisoners with military experience who want to fight for the country, authorities said.

Pentagon officials said that Russian troops are being slowed by Ukrainian resistance, fuel shortages and other logistical problems, and that Ukraine’s air defense systems, while weakened, are still operating.

But a senior U.S. defense official said that will probably change: “We are in day four. The Russians will learn and adapt.”

Putin hasn’t disclosed his ultimate plans, but Western officials believe he is determined to overthrow Ukraine’s government and replace it with a regime of his own, redrawing the map of Europe and reviving Moscow’s Cold War-era influence.

The fighting in southern Ukraine appeared aimed at seizing control of the country’s coastline. Cutting Ukraine’s access to its sea ports would deal a major blow to the country’s economy.

Ukrainian military deputy commander Lt. Gen. Yevhen Moisiuk sounded a defiant note in a message aimed at Russian troops.

“Unload your weapons, raise your hands so that our servicemen and civilians can understand that you have heard us. This is your ticket home,” Moisiuk said in a Facebook video.

As of Sunday, 368,000 people have fled Ukraine have arrived in neighboring countries since the invasion started Thursday, according to the United Nations Refugee Agency.

The agency added that the number “continues to rise.”

Ukraine’s U.N. ambassador, Sergiy Kyslytsya, tweeted Saturday that Ukraine appealed to the International Committee of the Red Cross “to facilitate repatriation of thousands of bodies of Russian soldiers.” An accompanying chart claimed 3,500 Russian troops have been killed.

Over the weekend, the U.S. pledged an additional $350 million in military assistance to Ukraine, including anti-tank weapons, body armor and small arms. Germany said it would send missiles and anti-tank weapons.

The U.S., European Union and Britain also agreed to block selected Russian banks from the SWIFT system, which moves money around thousands of banks and other financial institutions worldwide. They also moved to slap restrictions on Russia’s central bank.

Putin sent troops into Ukraine after building up a force of almost 200,000 troops along the country’s borders. He claims the West has failed to take seriously Russia’s security concerns about NATO, the Western military alliance that Ukraine aspires to join. But he has also expressed scorn about Ukraine’s right to exist as an independent state.

Russia claims its assault on Ukraine is aimed only at military targets, but bridges, schools and residential neighborhoods have been hit.