U.S. Secretary of State John Kerry condemned an attack by pro-Russian rebels on civilian neighborhoods in eastern Ukraine that killed at least 30 people Saturday. “I join my European counterparts in condemning in the strongest terms today’s horrific assault by Russian-backed separatists on civilian neighborhoods in Mariupol,” Kerry said in the statement, threatening increased international pressure on Russia.

“We call on Russia to end its support for separatists immediately, close the international border with Ukraine, and withdraw all weapons, fighters and financial backing,” Kerry added.

The latest attack in the nine-month-long conflict injured at least 83 others. European Union officials have also condemned the offensive and warn of further "grave deterioration" in relations with Russia, according to Reuters.

The Russian-backed separatists shelled more than 40 rounds of rockets from rebel-held territory into Mariupol. Smoke rose from homes, shops, schools and playgrounds after the attack in the city, which is nestled along the Sea of Azov. Women, children and the elderly were among the dead, according to the Organization for Security and Co-operation in Europe.

“It is reprehensible that the separatists are publicly glorifying this and other offensives in blatant violation of the Minsk agreements they signed,” Kerry said in the statement Saturday, adding Russia assisted in the attack by resupplying the rebels in east Ukraine with advanced weaponry including rockets and tanks.“Russia's commitment to the Minsk agreements it signed will be assessed by its actions not its words.”

Representatives of Ukraine, Russia and rebel groups signed the Minsk Protocol in September 2014 to stop fighting in the Donbass region of Ukraine. Mariupol, a city of about 500,000 people, is situated about 25 kilometers outside the ceasefire line, Kerry said.

However, rebel leader Alexander Zakharchenko has denied the Mariupol attack and Russia has long denied sending forces or weapons to war-torn east Ukraine, BBC News reported.

The rebels tried to capture the strategic port city last autumn before agreeing to a fragile ceasefire in September. Mariupol is along the coastal route from Crimea to the Russian border. Ukraine fears the separatists want to build a land bridge from Russia to Crimea, which was seized by Moscow last March.