There are reports of heavy fighting between Russian and Ukrainian forces around the city of Chernihiv, some 105 kilometers (about 65 miles) north of the capital of Kyiv.
Video from the city showed the regional headquarters of the Security Service of Ukraine on fire. A witness contacted by CNN said it was still burning late Friday.
The Russian defense ministry has acknowledged fighting in the area, saying Russian forces “have completed the blockade of the city of Chernihiv,” essentially cutting it off.
Ukraine says its forces inflicted heavy losses on Russian columns around Chernihiv, destroying more than thirty tanks.
Earlier Friday, the chief of Ukraine's armed forces, Valerii Zaluzhnyi, said the Ukrainian military had managed to hold the defense and repel the breakthrough of Russian troops in Chernihiv.
Chernihiv is on the main route from Belarus to Kyiv.
The Russian ground assault targeting the Ukrainian capital is developing from three directions.
Two are northern routes from Belarus, but the Ukrainians say they have slowed the Russian advance from the north-west after destroying a bridge at Ivankiv.
Another group of Russian armor has attacked Sumy in the northeast, which is also on a main road to Kyiv.