Two Russian missiles struck Kyiv during a visit by the head of the United Nations, Ukrainian officials said, but the West believes the battles for the besieged port of Mariupol and other areas in the east and south may determine the war’s outcome.

Russia withdrew its forces from outside Kyiv last month after failing to take the capital and launched a massive attack on Ukraine’s eastern Donbas region.

But Thursday’s blasts in Kyiv, heard soon after UN Secretary General Antonio Guterres completed talks with Ukraine’s President Volodymyr Zelenskiy, highlighted concerns that the capital remains vulnerable.

Zelenskiy said the blasts “prove that we must not drop our vigilance. We must not think that the war is over,” Reuters reported.

The rockets shook the central Shevchenko district of the city and one struck the lower floors of a 25-storey residential building, wounding at least 10 people, Ukrainian officials said.

Russian forces are now entrenched in the east, where Moscow-backed separatists have held territory since 2014, and are holding onto a swathe of the south that they seized in March.

Ukraine’s general staff said Russia was stepping up its military assault in the Donbas.

“The enemy is increasing the pace of the offensive operation. The Russian occupiers are exerting intense fire in almost all directions,” it said.

Putin calls Moscow’s actions a “special military operation” to disarm Ukraine, defend Russian-speaking people from persecution and prevent the United States from using the country to threaten Russia.

Ukraine dismisses Putin’s claims of persecution and says it is fighting an imperial-style land grab that has flattened Ukrainian cities, forced more than 5 million to flee abroad and killed thousands since the invasion started on Feb. 24.

Washington, which together with its allies has placed sweeping sanctions on Moscow, hopes Ukrainian forces can not only repel Russia’s assault in the east but also weaken its military so that it can no longer menace neighbours.