Russian missiles hit a medical clinic in the Ukrainian city of Dnipro, killing one, President Volodymyr Zelensky said, as Russia blamed Ukraine for dozens of strikes on its southern Belgorod region.
The regional governor in Belgorod said the Ukrainian military was responsible for dozens of artillery, mortar and drone attacks across the territory.
In Ukraine, a video distributed by officials showed a blaze ripping through a three-storey building in Dnipro after an attack that regional officials said had left some 23 people injured, including two children.
"We must defeat these inhumans irrevocably and as soon as possible. Because our time is our people. And our people are the most precious thing in Ukraine," Zelensky said in a statement.
Local media posted video footage of rescuers helping people with blood on their faces escape from the clinic through corridors full of rubble.
The head of the regional military administration Sergiy Lysak wrote earlier on Telegram that the Dnipropetrovsk region was "massively attacked" overnight "with rockets and drones".
Russian forces earlier this week targeted Dnipro in a late night strike with 16 missiles and 20 attack drones.
The industrial hub, which had nearly one million inhabitants before the war, lies around 125 kilometres (78 miles) from the current front line.
'Hostile environment'
A Russian strike on a residential building in Dnipro in January left more than 45 people killed and injured dozens more.
The announcement of the attack on Dnipro came as the governor of Russia's Belgorod region, Vyacheslav Gladkov said at least five districts of his region had been repeatedly hit by Ukrainian forces over the last 24 hours.
Five districts had been attack by drones, mortars and artillery and the village of Kozinka was struck more than 130 times he said.
The claims of fresh strikes in the southern Belgorod region come days after an unprecedented two-day incursion from Ukraine claimed by two armed groups, with Russia using its air force and artillery to push back the fighters.
Russia's defence ministry vowed an "extremely harsh" response to any further attacks on its soil.
The apparent exchange of artillery and missile fire reported Friday came one day after Belarusian strongman said Russia had begun the transfer of nuclear weapons to his country.
The Kremlin on Friday however stopped short of confirming the transfer had begun, saying only Russia's neighbour and ally was seeing "aggressive statements" from neighbouring countries.
"Belarus is in a very, very hostile environment," spokesman Dmitry Peskov told reporters.
In Washington, White House Press Secretary Karine Jean-Pierre earlier described Russia's move as "yet another example of making irresponsible and provocative choices".
China envoy in Moscow
Russia's foreign ministry meanwhile was expecting a visit to Moscow on Friday by China's special envoy for Ukraine, Li Hui as part of Beijing's latest effort to broker an end to the fighting.
Li's visit to the Russian capital, where he is expected to meet foreign minister Sergei Lavrov on Friday, comes after he met with Ukraine's Zelensky in Kyiv.
Earlier Friday, military officials in Kyiv said Russian forces launched overnight aerial attacks on the capital, adding that all the missiles were intercepted and destroyed.
"Another air attack on Kyiv, the 13th in a row since the beginning of May! And, as always, at nighttime," the city's military administration said on its Telegram account.
It said that Tu-95MS strategic bombers from the Caspian Sea region had launched cruise missiles at the city.
