Pro-Russian rebels launched an offensive against the strategic port of Mariupol in eastern Ukraine on Saturday, prompting the European Union's foreign policy chief to warn of a further "grave deterioration" in EU-Russian relations.
Mariupol's city administration said the rebels had killed at least 30 people and injured 83 others in the offensive by firing rockets from long-range GRAD missile systems.
The city of 500,000 on the Sea of Azov is vital for eastern Ukraine's steel and grain exports and also straddles the coastal route from the Russian border to Crimea, the Black Sea peninsula in southern Ukraine seized by Russia last March.
President Petro Poroshenko, pledging to protect Ukrainian territory, said he would convene an emergency meeting of his country's security council on Sunday.
"Today an offensive was launched on Mariupol. This will be the best possible monument to all our dead," Russia's RIA news agency quoted rebel leader Alexander Zakharchenko as saying at a memorial ceremony in the separatist-held city of Donetsk.
He said the separatists also planned to encircle Debaltseve, a town north-east of Donetsk, in the next few days, Interfax news agency quoted him as saying.
Eastern Ukraine has seen an escalation of fighting in recent days that Russian President Vladimir Putin has blamed on Kiev. The rebels have ruled out more peace talks.
Poroshenko responded angrily to the fighting in Mariupol, a city the rebels tried to capture last autumn before a fragile ceasefire was agreed in eastern Ukraine. Kiev fears the rebels want to build a land bridge from Russia to Crimea.
"We are for peace, but we accept the challenge of the enemy. We will protect our motherland," Poroshenko said in a statement.
EU foreign policy chief Federica Mogherini condemned the Mariupol attack and urged Moscow to lean on the rebel leaders.
"I ... call openly upon Russia to use its considerable influence over separatist leaders and to stop any form of military, political or financial support," she said.
Moscow denies sending forces and weapons to east Ukraine, despite what Kiev and the West say is irrefutable proof. Last week Poroshenko said Russia had 9,000 troops stationed in his country and demanded their immediate withdrawal.
The nine-month conflict, in which more than 5,000 people have been killed, has triggered the biggest crisis in relations between Russia and the West since the Cold War.
Saturday's attack on Mariupol started in the early morning, said pensioner Leonid Vasilenko, 76, who lives in an eastern suburb of the city.
"The walls were shaking, the window frames were shaking, paint started to crumble off the house," he said by telephone.
"I hid in the basement. What else can you do? I took the dog and the cat. In the basement you could hear the earth tremble."