A man walks past a burnt-out vehicle after a shelling by pro-Russian rebels of a residential sector in Mariupol, eastern Ukraine on Jan. 24, 2015. Fifteen people were killed in shelling in the east Ukrainian port city of Mariupol on Saturday, Ukraine's interior ministry said, an attack Kiev blamed on separatist rebels and the Russian military. Reuters

Ukraine’s President Petro Poroshenko said Sunday that intercepted radio conversations and telephone calls prove that pro-Russian separatists were responsible for firing rockets that killed at least 30 people in the city of Mariupol in eastern Ukraine on Saturday.

The attack on the strategically situated port city has prompted the European Union to warn of a further aggravation of relations with Russia, while U.S. President Barack Obama directly blamed Moscow for the deadly attack, and pledged to work with European allies to put more pressure on Russia, The Associated Press (AP) reported.

“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,” Poroshenko said, during an emergency meeting of Ukraine’s Security Council.

The Organization for Security and Co-operation in Europe’s (OSCE) special monitoring mission (SMM) also said that the rockets that pounded Mariupol were fired from areas that are controlled by rebels.

"The SMM conducted a crater analysis and its initial assessment showed that the impacts were caused by Grad and Uragan rockets… originated from a north-easterly direction, in the area of Oktyabr (19 km north-east of Olimpiiska Street), and the Uragan rockets from an easterly direction, in the area of Zaichenko (15 km east of Olimpiiska Street), both controlled by the “Donetsk People’s Republic,”" OSCE said, in a report.

Saturday’s attack occurred a day after separatist leader Alexander Zakharchenko had announced that his troops would increase their offensive against Kiev's forces in Mariupol. But, after reports of the extent of casualties began to appear, Zakharchenko denied responsibility for the attack and blamed the Ukrainian military for the killings, AP reported.

Meanwhile, the Russian foreign ministry said that the Ukrainian government should bear responsibility for the latest military escalation in eastern Ukraine, and that the current “situation is a result of Ukrainian troops crudely violating the Minsk agreements by constantly shelling residential settlements,” BBC News reported.

A peace deal, which was signed in September in the Belarussian city of Minsk, proposed a ceasefire and a withdrawal of heavy weapons from a demarcation line in eastern Ukraine. However, both sides have repeatedly violated the treaty since then.

“We are deeply concerned about the latest break in the ceasefire and the aggression that these separatists -- with Russian backing, Russian equipment, Russian financing, Russian training and Russian troops -- are conducting,” Reuters quoted Obama as saying at a news conference in New Delhi.

“I will look at all additional options that are available to us short of military confrontation and try to address this issue. And we will be in close consultation with our international partners, particularly European partners,” Obama said, adding that Washington has been considering all options -- except a military one -- to penalize Russia.