KYIV, UKRAINE, MARCH 01

Russian strikes pounded the central square in Ukraine's second-largest city and other civilian sites Tuesday in what the country's president condemned as a blatant campaign of terror by Moscow. "Nobody will forgive. Nobody will forget," vowed President Volodymyr Zelenskyy.

At the same time, a 40-mile convoy of hundreds of Russian tanks and other vehicles advanced on the capital, Kyiv, in what the West feared was a bid to topple Ukraine's government and install a Kremlin-friendly regime. And Russian President Vladimir Putin's forces 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 by tough economic sanctions that have thrown its economy its disarray and left the country practically friendless, apart from China and Belarus.

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 was hit. Explosions tore through residential areas, and a maternity ward was moved to an underground shelter.

Kharkiv's Freedom Square - Ukraine's largest plaza, and the nucleus of public life for the city - was struck with what was believed to be a missile, in an attack 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 strike blew out windows and walls of buildings that ring the massive central 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, representative of the Emergency Situations Ministry in Kharkiv region. In addition to the six killed, 20 were wounded in the strike, he said.

Zelenskyy called the attack on Kharkiv's main square "frank, undisguised terror," blaming a Russian missile and calling it 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 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.

In a worrying development, Human Rights Watch said it documented a cluster bomb attack outside a hospital in Ukraine's east in recent days. Local residents have also reported the use of the munitions in Kharkiv and the village of Kiyanka, though there was no independent confirmation.

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

The Kremlin denied Tuesday that it has used such weapons and insisted again that its forces only have struck military targets - despite evidence documented by Associated Press reporters of shelling of homes, schools and hospitals.

The International Criminal Court's chief prosecutor said earlier this week that he plans to open an investigation into possible war crimes.

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

The first talks Monday between Ukraine and Russia yielded no stop in the fighting, though the two sides agreed to another meeting in the coming days.

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 UN 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."

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

One after the other, explosions burst through a residential area of Kharkiv in one 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 convoy was 25 kilometers (17 miles) from the center of the city and stretched about 65 kilometers (40 miles), according to satellite imagery from Maxar Technologies.

Flames shot up from a military base northeast of Kyiv, in the suburb of Brovary, in footage taken from a car driving past. In another video verified by AP, a passenger pleaded with the driver, "Misha, we need to drive quickly as they'll strike again."

And Ukrainian authorities released details and photos of an attack Sunday on a military base in Okhtyrka, a city between Kharkiv and Kyiv, saying more than 70 Ukrainian soldiers were killed along with some local residents. The attack could not be immediately confirmed.

The Russian military's movements have been stalled by fierce resistance on the ground and a surprising inability to dominate Ukraine's airspace.

Ukrainians used resourcefulness 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 Russian military convoys. In Kyiv, sandbags were heaped in front of doors and windows of City Hall.