KYIV, Ukraine (AP) — A senior Ukrainian official says Russian forces on Saturday shelled a civilian evacuation convoy in the country’s northeast, killing 20 people. Bombardments have intensified as Moscow illegally annexed a swath of Ukrainian territory in a sharp escalation of the war.

Kharkiv region Gov. Oleh Syniehubov said the convoy was struck in the Kupiansy district, calling the attack on people who were trying to flee the area to avoid being shelled “сruelty that can’t be justified.”

Russian forces have not acknowledged or commented on the attack, apparently the second in two days to hit a humanitarian convoy. Russian troops have retreated from much of the Kharkiv region after a successful Ukrainian counteroffensive last month but continued to shell the area.

The attack comes at a pivotal moment in Russian President Vladimir Putin’s war. Facing a Ukrainian counteroffensive, Putin this week heightened threats of nuclear force and used his most aggressive, anti-Western rhetoric to date.

Ukrainian President Volodymyr Zelenskyy and his military vowed to keep fighting to liberate the annexed regions and other Russian-occupied areas.

Ukrainian officials said Saturday that their forces had surrounded thousands of Russian forces holding the strategic eastern city of Lyman, which is located in one of the four incorporated areas. Zelenskyy formally applied Friday for Ukraine to join NATO, increasing pressure on Western allies to help defend the country.