Ukraine's president was planning an emergency meeting of his military officials after a rebel rocket attack early Saturday morning killed 29 people in the port city of Mariupol, Fox News reported.
President Petro Poroshenko intended to rush back home from Saudi Arabia, where he was to attend the funeral of King Abdullah, to coordinate a reaction to the attack, Ukrainian officials said.
Ukraine's top rebel leader announced Saturday that an offensive had begun on the strategically important port.
On Friday, the rebels rejected a peace deal and said they were going on a multi-prong offensive against the government in Kiev to vastly increase their territory. The move is upending European attempts to mediate an end to the fighting.
Mariupol, which lies on the Azov Sea, is the major city between mainland Russia and the Russia-annexed Crimean Peninsula. Heavy fighting in the region in the autumn raised fears that Russian-backed separatist forces would try to establish a land link between Russia and Crimea.