POLTAVA, Ukraine — Funeral services were held Saturday for victims of one of the deadliest Russian airstrikes since the war in Ukraine began, as the heads of the British and American foreign intelligence agencies said Ukraine’s surprise incursion into Russia is a significant achievement that could change the narrative of the grinding 2 1/2-year war.

They also urged Kyiv’s allies not to be held back by Russia’s threats of escalation.

Richard Moore, the head of MI6, said Kyiv’s surprise August offensive to seize territory in Russia’s Kursk region was “typically audacious and bold on the part of the Ukrainians, to try and change the game.” He said the offensive — which Ukraine said has captured about 500 square miles of Russian territory — had “brought the war home to ordinary Russians.”

Speaking alongside Moore at an unprecedented joint public event Saturday in London, CIA Director William Burns said the offensive was a “significant tactical achievement” that had exposed vulnerabilities in the Russian military.

Burns said the West should be “mindful” of the escalation risk but not be “unnecessarily intimidated” by Russian saber-rattling, revealing that there was a moment in late 2022 when there was a “genuine risk of the use of tactical nuclear weapons” by Moscow.

In Ukraine, the funerals took place in the eastern city of Poltava for the victims of a Russian missile attack on a military training facility that left over 50 dead and more than 300 injured.

Hundreds of mourners, including grieving families, local residents and officials, gathered at the Cathedral of the Assumption in the city, some 200 miles southeast of Kyiv.

An air raid siren sounded during the service.