(ANSA-AP) - TRIESTE - Local authorities in the separatist stronghold in east Ukraine say 13 people have been killed after a city bus was hit by an artillery shell. The shell struck the vehicle Thursday morning in Donetsk, instantly killing numerous passengers and blowing out the windows of a nearby building.

Fighting between Ukrainian government troops and separatist forces surged after the New Year following a month of relative tranquility after a truce was declared in early December.

Residential areas are frequently struck as a result of artillery duels between the warring sides Ukrainian forces have all but abandoned Donetsk airport after months of bitter battle, although military officials continue to insist they hold part of the terminal. The Defense Ministry said Thursday that six soldiers had died over the previous day of fighting for the airport, which lies on the northern edge of the Russian-backed separatists' stronghold. Russian television reporters on Wednesday toured the airport building and showed the bodies of killed Ukrainian soldiers. Fighting for the airport peaked over the weekend, shattering the relative tranquility in place since a new truce was reached in early December. Kiev said six soldiers died and a further 16 were injured and taken prisoner during heavy fighting at the airport prior to the withdrawal.

Fighting in eastern Ukraine has killed more than 4,700 people since April, according to U.N. estimates.