Thousands of Ukrainians began fleeing the cities in the eastern part of the country, as Ukraine’s President Petro Poroshenko vowed to take revenge on pro-Russian separatists for killing almost 30 troops and injuring about 93 soldiers with rocket fire in Donetsk on Friday.

The Donetsk mayor’s office estimated 30,000 people have evacuated the city, but Alexander Boroday, the prime minister of the Donetsk People’s Republic, estimated that nearly 70,000 people either have fled or are fleeing, according to the Associated Press. The pro-Moscow separatists started the attack by firing rockets into Donetsk, located near the Russian border, making it one of the deadliest attacks by the rebels.

“For every life of our soldiers, the militants will pay with tens and hundreds of their own,” AP quoted Poroshenko as saying. He added: “Not one terrorist will evade responsibility. Everybody will get what is coming to them.”

Friday’s attack was a major setback for government troops as they recently have been successful in taking back from the rebels big chunks of territory in the country’s eastern reaches. Porosheko reacted to the attack by saying he had a “nasty surprise” ready.

Russian officials have claimed more than 500,000 people have fled Ukraine into their country since the beginning of the year. About 180,000 of them are registered as refugees, and more than 20,000 have applied for asylum in Russia, according to official estimates cited by ABC News. Meanwhile, Yulia Gorbunova, a Human Rights Watch researcher, told ABC News she estimated 100,000 Ukrainians have been internally displaced within their own country. Crumbling real-estate prices are among the results of this massive dislocation, the news outlet noted.

“Life has become impossible. Every day they shoot. All the windows in our apartment are shot out. The children have stopped sleeping,” said Irina Khodyko, a 38-year-old accountant quoted by AP who was leaving Donetsk for Kiev with her 12-year-old daughter and 10-year-old son. She added: “The Ukrainian army is in no rush to liberate us. They have just abandoned us, and we are forced to get ourselves out of this mess.”

After Poroshenko’s warning to the separatists, train tickets in Donetsk sold out, while most of the shops in the region shut down, according to the New York Times.