Second phase of war has started, says Ukraine president's chief of staff

"Believe in our army, it is very strong," chief of staff Andriy Yermak wrote on the Telegram messaging app, assuring Ukrainians that Ukraine's forces could hold off the offensive.

Ukraine's presidential chief of staff said on Monday that "the second phase of the war has started," referring to Russia's new assault in eastern Ukraine. His words echoed those of Ukraine's top security official, who said earlier that Russia had launched its new offensive on Monday morning.

A military vehicle is seen in front of a damaged building in Trostyanets, as Russia's invasion of Ukraine continues. (via REUTERS)

"Believe in our army, it is very strong," chief of staff Andriy Yermak wrote on the Telegram messaging app, assuring Ukrainians that Ukraine's forces could hold off the offensive.

Russian shelling in Ukraine's eastern Donetsk region killed four people on Monday, regional governor Pavlo Kirilenko said on his Telegram channel.

Russia has intensified attacks on several Ukrainian cities, but says it is concentrating its efforts on securing full control over eastern Ukraine's Donbass region -- made up of Donetsk and Luhansk regions.