After a day of punishing air strikes and fierce fighting with pro-Russia separatist gunmen, Ukraine said on Tuesday that it had regained control of Donetsk airport.
Dozens of militants were reported killed in the unusually lethal assault by Ukrainian government forces.
Reuters journalists counted 20 bodies in combat fatigues in one room of a city morgue in Donetsk. Some of the bodies were missing limbs, a sign that the government had brought to bear overwhelming firepower against the rebels for the first time.
“From our side, there are more than 50 [dead],” the prime minister of the rebels' self-styled Donetsk People's Republic, Alexander Borodai, told Reuters at the hospital.
Obama congratulates Poroshenko
The fighting for control of the industrial city erupted just hours after President-elect Petro Poroshenko celebrated victory in Sunday's presidential polls and vowed to defeat the armed separatists militarily.
On Tuesday, President Barack Obama telephoned Poroshenko to congratulate him on his election win.
The White House said Obama offered "the full support of the United States" in moving the country forward.
Obama and Poroshenko will reportedly meet in Europe next week but no venue has yet been announced.
Calls for dialogue
Russia called for an immediate halt to Ukrainian military operations in the east, and a "peaceful dialogue."
In response, Ukraine's First Deputy Prime Minister Vitaly Yarema said the military push will continue until "not a single terrorist remains on the territory of Ukraine."
Voicing alarm about continuing violence in Ukraine, United Nations Secretary-General Ban Ki-moon, and is urging the Kyiv government to use "exclusively peaceful means" to regain control of the country's troubled east.
The Organization for Security and Cooperation in Europe (OSCE) said Tuesday it had lost contact with four of its civilian monitors who were detained at a checkpoint near Donetsk.
OSCE spokesman Michael Bociurkiw told reporters Tuesday that the officials were traveling between cities in eastern Ukraine when they dropped out of contact.
Last month, seven OSCE monitors and their Ukrainian assistants were seized by separatists near the nearby city of Slovyansk. They were released unharmed about two weeks later.
Some reporting by Reuters