Several Ukrainian cities were targeted by a deadly onslaught of attacks from Russia on Monday, including the downtown area of Kyiv, where at least eight people were murdered, news agency AP reported.
Later, Russian President Vladimir Putin claimed that Kyiv's "terrorist" actions, notably an assault on a bridge leading to the Crimean Peninsula under Moscow's control, were the cause of the airstrikes on Ukraine.
"Crimea bridge blast is an act of terrorism; Ukraine's special forces behind the bridge attack. Ukraine has also tried to blow up Turkish Stream pipeline... if attacks continue against Russia, the response will be harsh," Putin said.
He said that Moscow will respond in a "tough and appropriate to the degree of threats" manner if Ukraine continues to carry out "terrorist acts" against Russia.
The prolonged, furious assault signalled a swift military escalation by Moscow. It happened the day after Putin described the explosion that occurred on Saturday on the substantial bridge that connects Russia to the Crimean peninsula that it has seized as a "terrorist attack" planned by Ukrainian special forces.
According to Kyiv, Russia started firing 75 missiles at Ukraine on Monday.
Around 8:15 a.m. local time (05:15 a.m. GMT), explosions erupted in Kyiv, and an AFP reporter in the city observed a large number of ambulances moving in that direction. On Monday morning, Kyiv heard at least five explosions.
"Several explosions in the Shevchenkivskyi district -- in the centre of the capital," Kyiv mayor Vitali Klitschko said on social media.
Ukraine's energy infrastructure was the target of Russian attacks: Zelensky Videos shared on social media showed black smoke spouting above a number of metropolitan neighbourhoods.
Russia launched its most recent attack on Kyiv, thus killing five people.
The blasts occurred a day after Moscow attributed the explosion on the bridge connecting Crimea to Russia, which claimed the lives of three people, on Ukraine.
(With Inputs From Agencies)