Ukraine's leader says intercepted radio and telephone conversations prove that Russian-backed separatists were responsible for firing the rockets that pounded the southeastern city of Mariupol and killed at least 30 people.

President Petro Poroshenko, who held an emergency meeting Sunday of his Security Council, said the intercepted calls offered irrefutable evidence that separatists committed the attack.

Separatist leader Alexander Zakharchenko initially announced that his forces were launching an offensive on Mariupol. But after the extent of civilian casualties became known, he backtracked and blamed Ukrainian forces for Saturday's carnage.

In Mariupol on Sunday, emergency workers recovered fragments of rockets from the scene of the killings. Representatives of the United Nations refugee agency handed out blankets to people left homeless or without heat because of the attack.