KIEV, Ukraine — Ukraine’s president said Sunday that intercepted radio and telephone conversations prove that Russia-backed separatists were responsible for firing the rockets that pounded the southeastern port city of Mariupol and killed at least 30 people.

The attack alarmed the West and looked likely further to aggravate relations with Russia.

Putting the blame squarely on Moscow, President Barack Obama said the U.S. would work with its European partners to “ratchet up the pressure on Russia.”

European Union foreign affairs chief Federica Mogherini said that EU foreign ministers would hold an “extraordinary” meeting in Brussels on Thursday to discuss Ukraine.