Ukrainian President Petro Poroshenko is vowing revenge tonight after a missile strike by eastern rebels struck a military base, killing 23 soldiers and wounding nearly 100 others.

The rebels fired rockets on a base at Zelenopillya with a BM-21 Grad, a multiple launcher rocket system common to Soviet bloc countries in the late 1960s. It is one of the deadliest rebel strikes in the ongoing conflict.

Ukraine’s Interior Ministry dubbed the incident a “terrorist act,” declaring the rebels to be “bloodthirsty scum‘ for striking a military target during what is basically open warfare in Ukraine’s east.

Russia continues to press the UN Security Council to come up with a plan for a ceasefire, though Ukraine’s government has ruled out any further cessations without an unconditional rebel surrender, and the US is loudly endorsing Ukraine’s offensive against the mostly ethnic Russian rebels.