Ukraine war live updates: Ukraine air strike sets Russian oil depot ablaze; China not expected to attend Swiss peace talks

This is CNBC's live blog tracking developments on the war in Ukraine. See below for the latest updates.

Russian missiles hit several locations in Ukraine's northeastern Kharkiv region overnight, killing five people and injuring at least 25, according to the regional head.

Oleh Syniehubov, head of Kharkiv's military administration, described the evening as a "difficult night for Kharkiv."

Elsewhere, two people were injured after a Ukrainian air strike set multiple fuel tanks ablaze at an oil depot in southern Russia, local officials said Friday.

Fedor Babenkov, the head of the Temryuk district in the Krasnodar region, said in a Google-translated post via Telegram that the fire has since been extinguished.

CNBC could not independently verify developments on the ground.

China has reportedly turned down the chance to attend Ukraine's peace conference in Switzerland next month.

Beijing has declined an invitation to join the June 15-16 summit because the conditions necessary for them to participate were not met, Reuters reported, citing three unnamed sources with direct knowledge of the matter.

Russia's Dmitry Medvedev on Friday said the Kremlin isn't bluffing over its threats to use tactical nuclear weapons against Ukraine, warning that its conflict with the West could escalate into what he described as "the worst possible scenario."

In a Google-translated post published on his official Telegram channel, Russia's former president said the use of tactical nuclear weapons can be "miscalculated" but that the Kremlin's position on the potential deployment of these weapons was not "not intimidation or nuclear bluff."

Medvedev, who currently serves as deputy chairman of Russia's influential Security Council, was president of