Russian forces kept up the shelling on Tuesday against civilian areas of Ukraine’s second-largest city, Kharkiv, as a long line of military vehicles moved closer to the capital Kyiv on the sixth day of Moscow’s military campaign.

Capturing the city of 1.5 million people is strategically important to Russia, experts say, and troops kept up a sustained attack on the city, which is located about 250 miles east of Kyiv.

For days, Russian forces have made slow progress toward the Ukrainian capital while shelling cities and firing missiles into strategic areas populated by thousands of civilians. On Tuesday a convoy of military vehicles about 40 miles long continued its march toward the capital.