Russian forces stepped up their attacks on crowded urban areas Tuesday, bombarding the central square in Ukraine’s second-biggest city and Kyiv’s main TV tower in what Ukraine’s president called a blatant campaign of terror.

“Nobody will forgive. Nobody will forget,” President Volodymyr Zelenskyy vowed after the bloodshed on the square in Kharkiv.

Ukrainian authorities said five people were killed in the attack on the TV tower, which is a couple of miles from central Kyiv and a short walk from numerous apartment buildings. Officials said a TV control room and a power substation were hit, and at least some Ukrainian channels briefly stopped broadcasting.

Zelenskyy’s office also reported a powerful missile attack on the site of the Babyn Yar Holocaust memorial, near the tower.

At the same time, a 40-mile convoy of hundreds of Russian tanks and other vehicles advanced slowly on Kyiv in what the West feared was a bid by Russian President Vladimir Putin to topple Ukraine’s government and install a Kremlin-friendly regime.

Many military experts worry that the Kharkiv attacks mean Russia could be shifting tactics. Moscow’s strategy in Chechnya and Syria was to use artillery and air bombardments to pulverize cities and crush fighters’ resolve.

Russian forces also pressed their attack on other towns and cities across the country, including the strategic ports of Odesa and Mariupol in the south.

Day 6 of the biggest ground war in Europe since World War II found Russia increasingly isolated, beset by tough sanctions that have thrown its economy into turmoil and left the country practically friendless, apart from a few countries like China, Belarus and North Korea.

The attack on the TV tower came after Russia’s Defense Ministry announced it would target transmission facilities in the capital used by Ukraine’s intelligence agency. It urged people living near such places to leave their homes.

Overall death tolls from the fighting remained unclear, but a senior Western intelligence official, who had been briefed by multiple intelligence agencies, estimated Tuesday that more than 5,000 Russian soldiers had been captured or killed so far.

Britain’s Ministry of Defense said it had seen an increase in Russian air and artillery strikes on populated urban areas over the past two days. The ministry also said three cities — Kharkiv, Kherson and Mariupol —were encircled by Russian forces.

In Kharkiv, Ukraine’s second-biggest city, with a population of about 1.5 million, at least six people were killed when the region’s Soviet-era administrative building on Freedom Square was hit with what was believed to be a missile.

The attack on Freedom Square — Ukraine’s largest plaza, and the nucleus of public life in the city — was seen by many Ukrainians as brazen evidence that the Russian invasion wasn’t just about hitting military targets but also about breaking their spirits.

The bombardment blew out windows and walls of buildings that ring the massive square, which was piled high with debris and dust. Inside one building, chunks of plaster were scattered, and doors, ripped from their hinges, lay across hallways.

“People are under the ruins. We have pulled out bodies,” said Yevhen Vasylenko, an emergency official.

Zelenskyy pronounced the attack on the main square “frank, undisguised terror” and a war crime. “This is state terrorism of the Russian Federation,” he said.

In an emotional appeal to the European Parliament later, Zelenskyy said: “We are fighting also to be equal members of Europe. I believe that today we are showing everybody that is what we are.”

He said 16 children had been killed around Ukraine on Monday, and he mocked Russia’s claim that it is going after only military targets.

“Where are the children, what kind of military factories do they work at? What tanks are they going at, launching cruise missiles?” Zelenskyy said.

Human Rights Watch said it documented a cluster bomb attack outside a hospital in Ukraine’s east in recent days. Local residents also reported the use of the weapons in Kharkiv and the village of Kiyanka, though there was no independent confirmation. The Kremlin denied using such weapons.

If the allegations are confirmed, that would represent a new level of brutality in the war and could lead to even further isolation of Russia.

Unbowed by Western condemnation, Russian officials upped their threats of escalation, days after raising the specter of nuclear war. A top Kremlin official warned that the West’s “economic war” against Russia could turn into a “real one.”

Throughout the country, many Ukrainian civilians spent another night huddled in shelters, basements or corridors. More than a half-million people have fled the country, and the U.N. human rights office said it has recorded the deaths of 136 civilians. The real toll is believed to be far higher.

“It is a nightmare, and it seizes you from the inside very strongly. This cannot be explained with words,” said Kharkiv resident Ekaterina Babenko, taking shelter in a basement with neighbors for a fifth straight day. “We have small children, elderly people and frankly speaking it is very frightening.”

U.N. humanitarian coordinator Martin Griffiths said the bombings had damaged water pipes and electrical lines. “Hundreds of thousands of families are without drinking water,” he said.

A Ukrainian military official said Belarusian troops joined the war Tuesday in the Chernihiv region in the north, without providing details. But just before that, Belarusian President Alexander Lukashenko said his country had no plans to join the fight.

In Kharkiv, explosions burst one after another through a residential area in a video verified by the AP. In the background, a man pleaded with a woman to leave, and a woman cried.

Determined for life to go on despite the attacks, hospital workers transferred a Kharkiv maternity ward to a bomb shelter. Amid makeshift electrical sockets and mattresses piled up against the walls, pregnant women paced the crowded space, accompanied by the cries of dozens of newborns.

Russia’s goals in hitting central Kharkiv were not immediately clear. Western officials speculated that it is trying to pull in Ukrainian forces to defend the city while a larger Russian force encircles Kyiv.

Russian troops continued their advance toward the capital, a city of nearly 3 million. The leading edge of the convoy was 17 miles from the center of the city, according to satellite imagery from Maxar Technologies.

A senior U.S. defense official described the long convoy as “bogged down,” saying Russia appeared to be pausing and regrouping to re-evaluate how to retake the momentum in the fighting.

Overall, the Russian military’s movements have been stalled by fierce resistance on the ground and a surprising inability to completely dominate Ukraine’s airspace.

The immense convoy, packed together along narrow roads, would seemingly be “a big fat target” for Ukrainian forces, the senior Western intelligence official said on condition of anonymity.

“But it also shows you that the Russians feel pretty comfortable being out in the open in these concentrations because they feel that they’re not going to come under air attack or rocket or missile attack,” the official said.

Ukrainians used whatever they had on hand to try to stop the Russian advance: On a highway between Odesa and Mykolaiv in southern Ukraine, residents piled tractor tires filled with sand and topped with sandbags to block convoys.