KYIV, Ukraine (AP) - Russian shelling pounded the central square in Ukraine’s second-largest city and other civilian targets Tuesday and a 40-mile convoy of tanks and other vehicles threatened the capital - as Ukraine’s embattled president accused Moscow of resorting to terror tactics to press Europe’s largest ground war in generations.

With the Kremlin increasingly isolated by tough economic sanctions that have tanked the ruble currency, Russian troops advanced on Ukraine’s two biggest cities. In strategic Kharkiv, an eastern city with a population of about 1.5 million, videos posted online showed explosions hitting the region’s Soviet-era administrative building and residential areas. A maternity ward relocated to a shelter amid shelling.

Ukrainian President Volodymyr Zelenskyy called the attack on Kharkiv’s main square “frank, undisguised terror,” blaming a Russian missile and calling it a war crime. “Nobody will forgive. Nobody will forget. ... This is state terrorism of the Russian Federation.”

As the fighting reached beyond military targets on Day 6 of a Russian invasion that has shaken the 21st century world order, reports emerged that Moscow has used cluster bombs on three populated areas. If confirmed, that would mean the war has reached a worrying new level.

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

The Russian defense minister vowed Tuesday to press the offensive until it achieves its goals, after a first round of talks between Ukraine and Russia yielded no stop in the fighting. Both sides agreed to another meeting in 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 U.N. human rights office said Tuesday that it has recorded the deaths of 136 civilians, including 13 children. The real toll is likely 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 Belarusian President Alexander Lukashenko said he had no plans to join the fight.

With Western powers sending weapons to Ukraine and driving a global squeeze of Russia’s economy, President Vladimir Putin’s options diminished as he seeks to redraw the global map - and pull Ukraine’s western-leaning democracy back into Moscow’s orbit.

“We are fighting also to be equal members of Europe,” Zelenskyy said in an address to the European parliament. “I believe that today we are showing everybody that is what we are. ... We have proven that, as a minimum, we are the same as you.”

On Monday, he said Kyiv was not prepared to make concessions “when one side is hitting another with rocket artillery.”

As the talks along the Belarusian border wrapped up, several blasts could be heard in the capital, and Russian troops advanced on the city of nearly 3 million. The convoy of armored vehicles, tanks, artillery and support vehicles was 17 milesfrom the center of the city and stretched about 40 miles, according to satellite imagery from Maxar Technologies.

“They want to break our nationhood, that’s why the capital is constantly under threat,” Zelenskyy said, saying that it was hit by three missile strikes on Monday and that hundreds of saboteurs were roaming the city.

Kharkiv, near the Russian border, is another key target. One after the other, explosions burst through a residential area of the city in one video verified by AP.

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 Kharkiv while a larger Russian force encircles Kyiv. They believe Putin’s overall goal is to overthrow the Ukrainian government and install a friendly one.

In a worrying development, Human Rights Watch has 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 near the northern city of Chernihiv, though there was no independent confirmation.