By Lidia Kelly and Olena Harmash

KYIV (Reuters) - Russia launched its largest air attack of the war on Ukraine overnight, setting the main government building on fire in central Kyiv and killing at least four people, including an infant, Ukrainian officials said on Sunday.

President Volodymyr Zelenskiy said the drone and missile barrage killed four people and caused damage across the north, south and east of the country, including the cities of Zaporizhzhia, Kryvyi Rih and Odesa, as well as in the Sumy and Chernihiv regions.

"Such killings now, when real diplomacy could have already begun long ago, are a deliberate crime and a prolongation of the war," Zelenskiy said in a post on X, issuing a fresh appeal to allies to strengthen Ukrainian air defences.

Just after sunrise, thick smoke could be seen rising into the clear blue sky from the burning top floor of the main government building, located in the historic Pecherskyi district, Reuters witnesses said.

Elsewhere in Kyiv, residential apartments were hit and damaged, with dozens of residents wrapped in blankets gathering on the streets outside to survey the damage to their homes as rescue workers fought to extinguish the flames.

The attack underlined growing pessimism in Ukraine and among allies that the war can be ended any time soon, with Russian President Vladimir Putin resisting calls for a ceasefire and emboldened by strengthening relations with China.

U.S. President Donald Trump said on Sunday he is ready to move to a second phase of sanctioning Russia, the closest he has come to suggesting he is on the verge of ramping up sanctions against Moscow or its oil buyers over the war in Ukraine. He did not elaborate.

Until now, Trump, who met Putin last month, has resisted imposing tougher sanctions on Russia.

U.S. Envoy to Ukraine Keith Kellogg said on Sunday that the Russian attack looks like an escalation in the conflict.

"The attack was not a signal that Russia wants to diplomatically end this war," Kellogg wrote on X.

U.S. Treasury Secretary Scott Bessent said on Sunday that additional economic pressure by the United States and Europe could prompt Putin to enter peace talks with Ukraine.

Zelenskiy said he spoke with French President Emmanuel Macron, coordinating diplomatic efforts, next steps and contacts with partners.

Kyiv's European allies have condemned the attack and vowed to stand by Ukraine politically and militarily, but concrete offers of assistance, including the possibility of troops on the ground, are still being discussed.

WAR'S BIGGEST DRONE BARRAGE

Ukrainian Prime Minister Yulia Svyrydenko said it was the first time in the war that the main government building in Kyiv had been hit, a symbolic blow to a well-defended part of the city.

Polish Prime Minister Donald Tusk said on X that the Russian attack on Kyiv’s government showed "again that the continued delaying (of) a strong reaction against Putin and the attempts to appease him made no sense".

Russia launched 805 drones against Ukraine overnight and 13 missiles, with Ukrainian air defence units downing 751 drones and four missiles, the air force said.

That was the highest number of drones Russia has used to attack the country since Moscow launched its full-scale invasion in February 2022.

Russia's defence ministry said it had carried out strikes on Ukraine's military-industrial complex and transport infrastructure, according to the TASS news agency. Both sides deny targeting civilians.

Timur Tkachenko, head of Kyiv's military administration, said an infant's body was pulled from the rubble in the Darnytskyi district, where a four-storey apartment building was damaged. A young woman was also killed in the attack on the district, which lies to the east of the Dnipro River, he said.

The interior ministry said more than 20 people were wounded in the capital. Air alerts lasted for more than 11 hours in Kyiv and the surrounding region.

In Kyiv's Sviatoshynskyi district in the west, several floors of a nine-storey residential building were partially destroyed. Falling drone debris set off fires in a 16-storey apartment building and two more nine-storey buildings, officials said.

UKRAINE TARGETS RUSSIAN ENERGY

Svyrydenko posted a video from inside the damaged government floor, showing a damaged roof, soot-stained ceilings and rescue workers cleaning the rubble.

"I urge the world to turn outrage over Russian crimes into concrete support for Ukraine," she said, standing on the damaged floor.

Ukraine's defence ministry said that a new meeting of Kyiv's allies was planned for next week and air defences and supplies for Kyiv's deep strikes on Russia would be discussed.

Ukraine's military said it attacked the Druzhba oil pipeline in Russia's Bryansk region, inflicting "comprehensive fire damage" during an overnight attack on Sunday.

It is part of a strategy to target Russia's vast energy complex, which is the financial backbone of its economy and helps fund the war.

Dozens of explosions also shook Ukraine's central city of Kremenchuk, cutting power to some residents and damaging a bridge across the Dnipro River, Mayor Vitalii Maletskyi said on Telegram.

Russian strikes on Kryvyi Rih, also in central Ukraine, targeted transport and urban infrastructure, city officials said, but no injuries were reported.

In the southern city of Odesa, civilian infrastructure and residential buildings were damaged, with fires breaking out in several apartment blocks, regional governor Oleh Kiper said. Three people were wounded, he said.

(Additional reporting by Pavel Polityuk, Yuliia Dysa, Valentyn Ogirenko in Kyiv and Lidia Kelly in Melbourne; Editing by Mike Collett-White, Alex Richardson and Matthew Lewis)