IRPIN, Ukraine (AP) — Moscow’s offensive in eastern Ukraine gathered momentum as several areas came under heavy shelling Thursday, amid suspicions Russian President Vladimir Putin wants to score a major battlefield success in time for Victory Day, one of Russia’s proudest holidays, on May 9.

Ukrainian authorities reported intense Russian fire in the Donbas — the eastern industrial heartland that the Kremlin is bent on capturing — and near Kharkiv, a northeastern city outside the Donbas that is seen as key to the offensive.

In the ruined southern port city of Mariupol, Ukrainian fighters holed up in the steel plant that represents the last pocket of resistance said concentrated bombing overnight killed and wounded more people. And authorities warned that a lack of safe drinking water inside the city could lead to outbreaks of deadly diseases.

The fresh attacks came as the United Nations chief met in Kyiv with Ukrainian President Volodymyr Zelenskyy and surveyed the destruction in small towns outside the capital that saw some of the worst horrors of the first onslaught of the war.

U.N. Secretary-General António Guterres condemned the atrocities committed in towns like Bucha, where evidence of mass killings of civilians was found after Russia retreated in the face of unexpectedly stiff resistance. He called such towns “the epicenter of unbearable heartbreak and pain.”

“Wherever there is a war, the highest price is paid by civilians,” Guterres lamented, reiterating the importance of investigating alleged war crimes.

Separately, Ukraine’s prosecutor accused 10 Russian soldiers, including a general, of being “involved in the torture of peaceful people” in Bucha. Prosecutor General Iryna Venediktova did not say her office had filed criminal charges, and she appealed to the public to help assemble evidence. Russia denies it targets civilians.

“During the occupation of Bucha, they took unarmed civilians hostage, killed them with hunger and thirst, kept them on their knees with hands tied and eyes taped, mocked and beat them,” Venediktova said.

Shortly after Zelenskyy and Guterres held a news conference, explosions shook Kyiv, and flames poured out of windows in at least two buildings in the capital city, which has been relatively unscathed in recent weeks. Plumes of smoke could be seen over the city. There was no immediate word on any casualties, but Guterres and his team were safe, a spokesperson said.

Getting a full picture of the unfolding battle in the east has been difficult because airstrikes and artillery barrages have made it extremely dangerous for reporters to move around. Several journalists have been killed in the war, now in its third month.

Also, both Ukraine and the Moscow-backed rebels fighting in the east have introduced tight restrictions on reporting from the combat zone.

Western officials say the Kremlin’s apparent goal is to take the Donbas by encircling and crushing Ukrainian forces from the north, south and east.

But so far, Russia’s troops and their allied separatist forces appear to have made only minor gains, taking several small towns as they try to advance in relatively small groups against staunch Ukrainian resistance.

Russian military units were mauled in the abortive bid to storm Kyiv and had to regroup and refit. Some analysts say the delay in launching a full-fledged offensive may reflect Putin’s decision to wait until his forces are ready for a decisive battle — rather than rushing in and risking another failure that could shake his rule amid worsening economic conditions at home because of Western sanctions.

Many observers expect Putin will try to claim a big victory in the east by Victory Day, which marks the defeat of Nazi Germany during World War II.

Putin, like many of his predecessors, often uses patriotic Russian holidays and anniversaries to make announcements, appeal for solidarity or show off his nation’s strength.

In March, he appeared at a Moscow stadium at a rally marking the eighth anniversary of Russia’s annexation of the Crimean Peninsula, and used the event to build support for the war.

The discovery of the mass killings around Kyiv helped to galvanize support for Ukraine in the West. Bulgarian Prime Minister Kiril Petkov vowed his country would join others in providing military assistance as he toured another scene of atrocities outside Kyiv, in Borodyanka.

“We cannot be indifferent. We cannot say that this is a Ukrainian problem. We cannot say some people are dying but we are not interested in that,” he said. “This is not just the battle for Ukraine, but it is a matter for civilization to choose which side to take.”

Bulgaria, under a new liberal government that took office last fall, has severed many of its old ties to Moscow and supported punitive measures against the Kremlin.

The visit by the Bulgarian leader came a day after Russia cut off the supply of natural gas to his country and fellow NATO member Poland, in what was seen as a bid to punish and divide the West.

As Russia presses its offensive, civilians again bear the brunt.

“It’s not just scary. It’s when your stomach contracts from pain,” said Kharkiv resident Tatiana Pirogova. “When they shoot during the day, it’s still OK, but when the evening comes, I can’t describe how scary it is.”

Ukraine’s military said that Russian troops were subjecting several places in the Donbas to “intense fire” and that over the past 24 hours, Ukrainian forces had repelled six attacks in the region.

Four civilians were killed in heavy shelling of residential areas in the Luhansk region of the Donbas, according to the regional governor.

Columns of smoke could be seen rising at different points across the Donetsk region of the Donbas, and artillery and sirens were heard on and off.

In Zaporizhzhia, a crucial waypoint for tens of thousands of people fleeing Mariupol, an 11-year-old boy was among at least three people wounded in a rocket attack that authorities said was the first to hit a residential area in this southern city since the war began. Shards of glass cut the boy’s leg down to the bone.

Vadym Vodostoyev, the boy’s father, said: “It just takes one second and you’re left with nothing.”

Satellite photos analyzed by The Associated Press also showed evidence of intense Russian fire on Mariupol in recent days.

A video posted online by Ukraine’s Azov Regiment inside the steel plant showed people combing through the rubble to remove the dead and help the wounded. The regiment said the Russians hit an improvised underground hospital and its surgery room, killing an unspecified number of people. The video couldn’t be independently verified.

Hundreds of thousands of Mariupol’s residents have fled. Authorities said the estimated 100,000 who remain run the risk of diseases like cholera and dysentery.

“Deadly epidemics may break out in the city due to the lack of centralized water supply and sewers,” the council said on the messaging app Telegram. It reported bodies decomposing under the rubble and a “catastrophic” shortage of drinking water and food.

Russia, meanwhile, said a city under its control in the south came under fire. In what may have been a Ukrainian counterattack, a series of explosions boomed near the television tower late Wednesday in Kherson, which has been occupied by Russian forces since early in the war. The blasts at least temporarily knocked Russian channels off the air.

Loud bangs were reported in Russia’s Belgorod region bordering Ukraine, but there was no immediate explanation. In recent days, fuel and ammunition sites on Russian soil have been hit with explosions and fires, and suspicion has fallen on Ukraine.

Ukraine has urged its allies to send even more military equipment to fend off the Russians. U.S. President Joe Biden plans to ask Congress for an additional $33 billion to help Ukraine.