Russian troop build-up threatens city seen as key to seizing Ukraine's Donbas

Russian troops have infiltrated the strategic city of Kostyantynivka in eastern Ukraine and are now trying to surround it. The entire city is now effectively in a "grey zone", no longer controlled by anyone, Ukrainian soldiers have told the BBC. "They get into areas behind our backs and in urban conditions it's extremely difficult to push them out," says a Ukrainian drone pilot who operates in that area and prefers to remain anonymous.Kostyantynivka is a gateway to the rest of the Donbas region.If it falls, Russian forces would be able push towards Ukraine's last remaining strongholds in the east, the cities of Kramatorsk and Sloviansk, and move closer to seizing Donbas completely, one of the Kremlin's key objectives in this war.For months Russia's full-scale war in Ukraine has stalled along the front line, and Ukrainian commanders say they have recaptured more territory this year than they have lost, disrupting Moscow's crucial supply lines between the Russian border and occupied Crimea. On Sunday, Russian-appointed authorities in Crimea suspended fuel sales to the public because of the shortages.And Kyiv's repeated strikes on oil refineries in Moscow, St Petersburg and other cities have also sought to bring the conflict to the attention of Russians far from the front line.