KYIV, Ukraine, Sept 8, 2025 (BSS/AFP) - US President Donald Trump threatened Sunday to impose more sanctions on Russia, after the Kremlin unleashed its biggest-ever aerial barrage at Ukraine.

Russian missiles and drones rained down across Ukraine early Sunday, killing four people and setting government offices in the capital Kyiv ablaze.

Trump told reporters after the assault he was "not happy with the whole situation" and said he was prepared to move forward on new sanctions on Moscow.

Ukrainian President Volodymyr Zelensky said he was counting on a "strong" US response.

Russia has intensified its onslaught against Ukraine since a meeting between Trump and President Vladimir Putin on August 15 failed to make any breakthrough on a ceasefire.

After Sunday's attack on Kyiv, flames could be seen rising from the roof of the sprawling government complex that houses Ukraine's cabinet of ministers in the heart of the city -- the first time it has been hit during the three-and-a-half-year conflict.

Drone strikes also damaged several high-rise buildings in the Ukrainian capital, according to emergency services.

Russia denies targeting civilians in Ukraine.

It said it struck a plant and a logistics hub in Kyiv, with the Russian defence ministry saying "no strikes were carried out on other targets within the boundaries of Kyiv".

"It is important that there is a broad response from partners to this attack today," said Zelensky in his evening address, adding that Putin was "testing the world".

"We are counting on a strong response from America. That is what is needed."

- 'Deliberate crime' -

Russia fired at least 810 drones and 13 missiles at Ukraine between late Saturday and early Sunday in a new record, according to the Ukrainian air force.

Ukrainian Prime Minister Yulia Svyrydenko posted a video showing a damaged floor in the government building.

"We will restore the buildings," she said. "But we cannot bring back lost lives. The enemy terrorises and kills our people every day throughout the country."

Zelensky discussed the attack in a call with French President Emmanuel Macron and said France would help Ukraine strengthen its defence.

Macron was among European leaders who condemned the attack, posting on X that Russia was "locking itself ever deeper into the logic of war and terror".

UK Prime Minister Keir Starmer denounced the attacks as "cowardly" while EU chief Ursula von der Leyen accused the Kremlin of "mocking diplomacy".

Earlier, US Treasury Secretary Scott Bessent said Washington might slap tariffs on countries that buy Russian oil.

"The Russian economy will be in full collapse. And that will bring President (Vladimir) Putin to the table," Bessent told NBC television.

- European troop proposal -

At least two people were killed in a strike west of Kyiv, prosecutors said.

More than two dozen were wounded in Kyiv, according to the emergency services.

Among them was a 24-year-old pregnant woman who delivered a premature baby shortly after the attack, with doctors fighting to save her life and that of her baby, state TV Suspilne reported.

Two more died and dozens were wounded in overnight strikes across the east and southeast, authorities said.

Ukraine's foreign ministry highlighted that seven horses had also been killed at an equestrian club.

"The world cannot stand aside while a terrorist state takes lives -- human or animal -- every single day," it posted on X.

The barrage came after more than two dozen European countries pledged to oversee any agreement to end the war, some of which said they were willing to deploy troops on the ground.

Ukraine has insisted on Western-backed security guarantees to prevent future Russian attacks, but Putin has warned that any Western troops in Ukraine would be unacceptable and legitimate targets.

Trump has tried to find a way to end the war in recent weeks but has little to show for his efforts.

Russia has continued to claim territory in costly grinding battles and now occupies around 20 percent of Ukraine.

Tens of thousands have been killed and millions forced from their homes in Europe's bloodiest conflict since World War II.