As fighting rages, Ukraine pushes for a cease-fire in talks, while sanctions hit Russia's economy

Ukrainian armed forces have set up checkpoints in cities, like the one shown here in Zhytomyr on Sunday.Viacheslav Ratynski / Reuters

Kyiv's position has been strengthened by its staunch early opposition, military assistance from countries around the world, and a growing global effort to economically isolate Russia.

On Monday, Russian and Ukrainian officials met for talks as fighting raged around key cities and Russian President Vladimir Putin faced the fallout from an invasion that has rallied the West behind his neighbor and against Moscow.

Ukraine said it would push for an immediate ceasefire and the withdrawal of Russian troops from the country ahead of the border talks with Belarus.

Kyiv's position has been bolstered by its staunch early opposition, military assistance from around the world, and a growing global sanctions campaign that has sent Moscow's economy reeling.

Despite the fact that officials were negotiating at the border, Ukraine said its second-largest city was being shelled by Russia.

The conflict is now being fought in the shadow of nuclear threats, with Russia's military declaring on Monday that its nuclear deterrent forces have been placed on high alert in response to Putin's order.

'A lot of determination'

While an increasing number of people have fled the Russian advance, those who have remained have been subjected to an air assault that Kyiv and international watchdogs have said is increasingly targeting civilians.

Russian forces have been accused of carrying out a massive shelling in Kharkiv, Ukraine's second-largest city, with shocking video purporting to show the assault.

Ukraine's interior ministry adviser Anton Gerashchenko said in a video that appeared to show flashes in the sky as smoke filled the air, "Kharkiv has just been massively shelled."

"Dozens of people have died, and hundreds have been injured!" This atrocity must be witnessed by the entire world! "Death to the intruders!" he yelled.

The video has been verified by NBC News, but no reports of casualties from the apparent incident have been confirmed. Russia has denied repeatedly that it is targeting civilians.

According to UN human rights chief Michelle Bachelet, 102 civilians, including seven children, have been killed in Ukraine since Thursday. She went on to say that these figures were most likely an undercount.

Meanwhile, the head of the United Nations refugee agency reported that more than 500,000 people had fled the country in the aftermath of the Russian attack.

Others have sought refuge inside metro stations and parking garages as cities have been hit by missile strikes and gunfire.

As Russian troops advanced on the capital, a 39-hour curfew that had kept people off the streets of Kyiv all weekend was lifted early Monday.

Residents went out Monday to restock supplies and waited in line at supermarkets while air raid sirens wailed.

"We are calm and do have a lot of resolve," Vadym Khortiuk, 28, said, adding that "we have a lot of support from our friends from the West."

This support has come in the form of increased pledges of weapons and other military equipment, as well as a concerted effort to make Putin pay for the invasion financially.

The ruble fell to record lows early Monday as a result of crippling sanctions imposed by the United States and its allies, while Russia's central bank closed the stock market and raised its interest rate to 20% from 9.5 percent in an attempt to shore up the plummeting national currency and prevent a bank run.

The United States increased the pressure by announcing that it would expand sanctions against the country's central bank, barring Americans from doing business with it and freezing any assets the bank has in the United States.

The State Department also announced that all operations at the embassy in Minsk, Belarus, had been halted due to "security and safety concerns." It also allowed non-emergency employees and their families to leave the embassy in Moscow on their own accord.

Dmitry Peskov, a Kremlin spokesman, acknowledged that "Russia's economic reality has changed" as a result of the "heavy" sanctions, but said the country was prepared and capable of dealing with the fallout. On Monday, Putin met with advisers to discuss the economy, and in remarks broadcast on state television, he referred to the West as a "empire of lies."

In retaliation for the bloc's ban on all flights from Russian planes over its airspace, Russia also closed its airspace to airlines from the European Union and several other countries.

Russia's progress has slowed

Ukrainian President Volodymyr Zelenskyy has expressed doubts about the outcome of the talks with Russia.

However, his country's forces have slowed Russia's advance and continue to hold the capital, Kyiv, as well as Kharkiv, near the Russian border, and other key cities where heavy fighting has occurred.

"The majority of Putin's ground forces remain more than 30 kilometers [18 miles] to the north of Kyiv," Britain's Ministry of Defense said in a Twitter update Monday morning. "Logistical failures and tenacious Ukrainian resistance continue to thwart Russia's advance," it added.

A satellite image, taken and released on Sunday by Maxar, appears to show part of a large convoy of Russian ground forces heading toward Kyiv from the northwest. NBC News has not verified the image.Maxar technologies / AFP - Getty Images

Officials in the country's southeast confirmed that Russian forces had taken control of Berdyansk as they moved along the coast toward Mariupol, a key port city considered a prime Russian target.

Zelenskyy praised Ukraine's military in a video statement released early Monday, claiming that 4,500 Russian troops have been killed in the conflict so far.

The number of people killed on either side has not been confirmed by NBC News.

Armed with government-issued machine guns, homemade Molotov cocktails, and shovels to build barricades, civilians have aided in the reinforcement of their army's desperate but for the time being effective resistance. They will soon be joined by prisoners with combat experience who have been released, according to Zelenskyy.

He also urged the EU to "immediately accept Ukraine."

According to Zelenskyy and Western officials, Putin intends to depose Ukraine's pro-Western government and possibly replace it with a Moscow-friendly regime, restoring influence as the Kremlin seeks to upend the continent's post-Cold War order.

The conflict has left Europe in the grip of its worst security crisis in decades.

Putin made that point even clearer on Sunday, when he issued a directive to increase the readiness of Russia's nuclear deterrent forces, citing NATO's "aggressive statements" and tough financial sanctions imposed by the United States, Europe, and others.

Russia, like NATO and the United States, possesses thousands of nuclear warheads.

Jens Stoltenberg, the secretary general of the transatlantic alliance, called the move "reckless and dangerous."

"There isn't any reason for that. NATO poses no danger to Russia "He told NBC News in an interview Monday morning in Brussels.

Although NATO increased its presence in Eastern Europe in the aftermath of the Ukraine invasion, he reiterated that the organization had no intention of becoming involved in the conflict itself.

"We have a responsibility to make sure this doesn't get out of hand," he said. "That's going to be extremely dangerous."