Intercepted radio and telephone conversations prove that Russian-backed separatists were responsible for firing the rockets that pounded Ukraine's southeastern city of Mariupol and killed at least 30 people, President Petro Poroshenko said Sunday during an emergency meeting of his Security Council. U.S. President Barack Obama also put the blame on Moscow, warning that the United States would work with European partners to "ratchet up the pressure on Russia."Separatist leader Alexander Zakharchenko initially announced that his forces had begun an offensive on the government-controlled 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 multi-pronged offensive against the Kiev 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 United Nations estimates.In Mariupol on Sunday, emergency workers disposed of rocket fragments at the scene of the attack. Police said two unexploded rockets were found in a bank and an apartment building.U.N. 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 Russian-annexed Crimea. 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.A peace deal signed in September 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 was "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."And we will continue to take the approach that we've taken in the past, which is to ratchet up the pressure on Russia and I will look at all additional options that are available to us short of military confrontation and try to address this issue."Obama said the U.S. would work "in close consultation with our international partners, and particularly European partners, to ensure that they stay in lock-step with us on this issue.""Russia is actively supporting the separatists by supplying them with heavy weaponry and vehicles, including tanks, armored personnel carriers, and heavy artillery pieces, as well as providing military personnel for exercising ongoing tactical support," the United States State Department said in a statement while indicating Russian involvement into the Ukraine crisis. "We again call on Russia to denounce its separatist patrons, withdraw all support to them, and stop the flow of heavy weapons, fighters, and advisors, and restore Ukraine's control along its side of the international border and allowing OSCE monitoring all along both sides of the border. Russia holds the keys to peacefully resolving a conflict it started and bears responsibility to end the violence which has devastated the lives of so many innocents in Donetsk and Luhansk," the statement continued.The months-long conflict in eastern Ukraine has come at the cost of an increasingly high death toll, with a large increase in the numbers of internally displaced people. "More than 630,000 Ukrainians have been displaced internally, and some 590,000 people have fled to neighboring countries. In addition, more than five million people are living within the conflict zone," the Norwegian Refugee Council reported, revealing the worst humanitarian crisis in Europe. The major humanitarian crisis hit the country in February 2014. Following pro-Russian rebels self-proclaiming the existence of the Donetsk People's Republic in April, instability has prevailed in eastern Ukraine. After declaring independence, clashes have increased throughout the region where government forces have stepped up operations.