As Ukraine buried the victims on Saturday of one of Russia's deadliest airstrikes, the heads of the U.S. and British foreign intelligence agencies said that ongoing support for Ukraine in its conflict with Russia is essential.

Hundreds of Ukrainians, including sobbing relatives, gathered at the Cathedral of the Assumption in Poltava to mourn the more than 50 dead and more than 300 injured in a Russian missile strike on a military training facility and a nearby hospital.

Late Friday, Ukrainian President Volodymyr Zelenskyy said 55 people were killed and 328 injured in the Tuesday attack in Poltava.

"That includes people with severe injuries, such as amputations and internal organ damage," Zelenskyy said, speaking at a conference outside the Italian city of Milan.

Zelenskyy, frustrated by Western allies' reluctance to remove restrictions on the use of the weapons they supply, said Ukraine is developing its own drones and missiles.

"Eventually, [Russian President Vladimir] Putin will feel the pressure to seek only one thing: peace," he said.

US, British intelligence publish statement

Meanwhile, in their joint opinion essay published Saturday in the Financial Times newspaper, CIA Director William Burns and Richard Moore, head of Britain's MI6 secret intelligence service, pledged to enhance cooperation between their agencies and address other global challenges.

"Staying the course in Ukraine is more vital than ever," they wrote. "Putin will not succeed in extinguishing Ukraine's sovereignty and independence." It marked the first time the leaders of the two agencies have co-written an article.

The spy chiefs also addressed Russian intelligence operations, accusing them of sabotage and disinformation across Europe. Russia has denied those allegations.

Russia launches drones

Overnight Friday into Saturday, Russia launched 67 drones into Ukraine, the Ukrainian Air Force said. Sixty-one were destroyed, it said. No injuries or serious damage were reported, but drone debris could be seen outside Ukraine's parliament building.

In the eastern city of Kostiantynivka, Donetsk region Governor Vadym Filashkin said an artillery attack killed four people and injured three more. It also damaged a high-rise building and power lines.

Ukraine launched its own strikes Saturday, targeting an ammunition depot in the Russian region of Voronezh, about 150 kilometers (90 miles) from Ukraine.

Governor Aleksandr Gusev said that Russia "neutralized a drone" but the debris fell, sparking a fire "that spread to explosive devices and caused them to detonate," prompting the evacuation of residents, he said. No one was injured, he added.

Russian Telegram channels said the fire broke out in a local ammunition depot, according to Agence France-Presse, adding that the Security Service of Ukraine, also known as SBU, later claimed it had hit a Russian ammunition depot.

Some information for this report was provided by The Associated Press, Reuters and Agence France-Presse.