KIEV — Ukraine’s president said Sunday intercepted radio and telephone conversations prove that Russia-backed separatists were responsible for firing the rockets that pounded the southeastern city of Mariupol and killed at least 30 people.
The attack on Mariupol, a strategically situated port city that had been relatively quiet for months, alarmed the West and appeared likely to further aggravate relations with Russia.
Putting the blame on Moscow, President Obama said the United States would work with its European partners to ‘‘ratchet up the pressure on Russia.’’
Federica Mogherini, European Union foreign affairs chief, revealed that EU ministers would hold an ‘‘extraordinary’’ meeting in Brussels on Thursday to discuss Ukraine.
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Foreign Minister Sergey Lavrov of Russia, speaking separately with Mogherini and Secretary of State John Kerry, told them the Ukrainian government bore responsibility for the latest military escalation, according to statements released by his ministry. Lavrov did not, however, directly address who had carried out the attack.
Separatist leader Alexander Zakharchenko initially said his forces had begun an offensive on the city of Mariupol. But after the extent of civilian casualties became known, he backtracked and blamed Ukrainian forces for Saturday’s carnage.
The rocket attack came a day after the rebels rejected a peace deal and announced they were going on a multipronged offensive against the government in Kiev in a bid to seize more territory. The rebel stance has upended European attempts to mediate an end to the fighting in eastern Ukraine that has cost at least 5,100 lives since April, according to UN estimates.
‘‘The intercepted radio and telephone conversations, which were given to me by Ukraine’s security services, irrefutably prove that the attack was conducted by the terrorists, who, unfortunately, are supported by Russia,’’ President Petro Poroshenko said during an emergency meeting of his Security Council.
The Organization for Security and Cooperation’s monitoring mission said Saturday the Grad and Uragan rockets that hit Mariupol were fired from areas under rebel control. The organization said its Permanent Council would meet Monday in Vienna ‘‘in light of the rapid deterioration of the situation in eastern Ukraine.’’
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In Mariupol on Sunday, emergency workers disposed of rocket fragments. Police said two unexploded rockets were found in a bank and an apartment building.
UN refugee agency workers handed out blankets to people left homeless or without heat because of the shelling, which hit schools, homes, and shops.
‘‘The city is in shock,’’ Mariupol resident Yelena Khorshenko said by telephone. ‘‘The streets are empty, and people are boarding up their windows and preparing for the worst.’’
Mariupol lies between Russia and the Russian-annexed Crimean Peninsula. Heavy fighting in the region in the fall raised fears that the Russian-backed separatists would try to capture the city to forge a land link between the two.
In Kiev, hundreds gathered on the central square in memory of those who died in Mariupol. In addition to the 30 people killed, 95 people were wounded.
A peace deal signed in September in Minsk envisaged a cease-fire and a pullout of heavy weapons from a division line in eastern Ukraine, but both sides have repeatedly violated the pact.
The United States is ‘‘deeply concerned about the latest break in the cease-fire and the aggression that these separatists with Russian backing, Russian equipment, Russian financing, Russian training, and Russian troops are conducting,’’ Obama said during a visit to New Delhi.
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