Russia has blocked a United Nations Security Council (UNSC) statement condemning an indiscriminate grad rocket attack Saturday on the Ukrainian port city of Mariupol that killed around 30 people and injured scores more.

With a population of nearly 500,000, government-held Mariupol is the largest city on the Azov coastal stretch that separates rebel-held territories from the Crimean peninsula annexed by Russia in March.

The proposed UNSC statement called for an "objective investigation" into the incident and noted that Alexander Zakharchenko, the prime minister of the so-called "Donetsk People's Republic" (DPR), announced the start of a new offensive shortly after the attack.

Russia's UN mission told Reuters that consensus on the council statement — which requires unanimous approval — had not been reached because the UK insisted on condemning Russian-backed "self-defense" forces. It also criticized Western council members for refusing to condemn "aggressive statements and actions by the Kiev government."

UN Secretary-General Ban Ki-moon issued a statement saying he "strongly condemns" the attack, and noting that the "rockets appear to have been launched indiscriminately into civilian areas, which would constitute a violation of international humanitarian law."

Russia also vetoed a UNSC resolution last year that declared a referendum on secession in Crimea illegal.

On Saturday, as bodies were still being retrieved from the rubble in Mariupol, Zakharchenko claimed responsibility for the assault and promised a broader attack was underway. "Today the offensive on Mariupol has begun… this is the best possible monument to our dead," he said, laying a wreath of flowers at a memorial service in Donetsk for the 13 people killed in a mortar attack on a trolleybus stop two days before. "God in a few days willing we will close the boiler of Debaltseve and avenge the killing of our people," he added to cheers from the crowd.

But just a few hours later in a bizarre interview with a Russian state-media television channel, the rebel leader retracted the boast and claimed exactly the opposite. "We are not beasts, we do not hide behind civilians… Mariupol will be liberated but not using the methods of Kiev," he told Russia 24.

Edward Basurin, assistant to the DPR defense minister, told VICE News that Ukrainian forces had shelled their own city and that separatist forces "had not launched any attack on Mariupol."

In a report released Saturday, Human Rights Watch field researcher Ole Solvang said that examination of rocket remnants and more than 20 craters spanning several blocks indicated that the missiles had been fired from position "due east" in the direction of the frontline — suggesting that the attack was launched from separatist territory.

According to Mariupol's local council, 67 residential blocks and four schools were struck in the attack.

Oleksander Turchynov, secretary of Ukraine's National Security and Defense Council, called the attack on Mariupol "a bloody crime against humanity." He said it was committed by "terrorists" under the "control of the army," and a "direct result of Putin's goals."

Violence has spiked across the region over the last week, with more than 262 people killed in the last week. According to the latest report from the United Nations, the fighting has displaced at least a million people, and, while the official death toll stands at 5,086, the true figure is likely "much higher."

As all eyes are on Mariupol, a fierce battle continues to rage for Debaltseve, a city 35 miles northwest of Donetsk. Shelling has hit several nearby villages, including Svitlodarsk and Uglegorsk. Thousands of soldiers in the government-held enclave have been surrounded on three sides by separatist forces for weeks. The only route in and out of the strategically important town, which sits on a transport hub, requires passing over a partially destroyed bridge.

A withdrawal would mean passing through a potentially deadly bottleneck, prompting fears that a retreat from the town could be another Iloviask — a catastrophic Ukrainian defeat that resulted in hundreds of casualties and the loss of more than 60 pieces of military equipment, including tanks and rocket launchers.

Speaking to Ukraine's Channel 5 after the attack on Mariupol, Yuriy Bereza, the commander of the Dnipro-1 Battalion that fought in Iloviask, called for the resignation of Viktor Muzhenko, the commander-in-chief of Ukraine's armed forces.

"Any decent person in the place of General Muzhenko would resign," Bereza said. "He has no strategic thinking."