NATO and the U.S. demanded that Russia stop supporting rebels in Ukraine after indiscriminate rocket fire Saturday slammed into a market, schools, homes and shops in the southeastern city of Mariupol, killing at least 30 people.

Ukrainian President Petro Poroshenko cut short a trip to Saudi Arabia and held an emergency meeting of his military officials as they rushed to defend the strategically important port, beefing up military positions with more equipment and sending in more forces.

The separatists’ top leader, Alexander Zakharchenko, declared that an offensive against Mariupol had begun, but later toned down his threats as the scale of the civilian casualties became clear.

Poroshenko pointed to Russia as the main provacateur, as he has repeatedly while the rebels’ firepower has grown.

“The time has come to name their sponsors. The help given to militants, weapons deliveries, equipment and the training of manpower — is this not aiding terrorism?” Poroshenko said.

Russia insists it does not support the rebels, but Western military officials say the sheer number of heavy weapons under rebel control belies that claim.

The rocket attacks came a day after the rebels rejected a peace deal and announced they were going on a multi-prong offensive against the government in Kiev to vastly increase their territory. The rebel stance has upended European attempts to mediate an end to the fighting, which the U.N. says has killed nearly 5,100 people since April.

Mariupol lies between mainland Russia and the Russia-annexed Crimean Peninsula. Heavy fighting in the region in the fall raised fears that Russian-backed separatist forces would try to capture the city to establish a land link between Russia and Crimea.

Ukraine’s Defense Ministry said three separate strikes from Grad multiple-rocket launchers hit Mariupol and its surrounding areas Saturday.

“The shelling was carried out by militants,” Mariupol Mayor Yuriy Khotlubei said. “This is very clearly Russian aggression that has caused terrible losses for the residents of the eastern part of our city.”

The Donetsk regional government said the dead included a 15-year old girl and a 5-year old boy.

The RIA Novosti news agency cited rebel leader Zakharchenko as saying an offensive had begun on Mariupol. He spoke as he laid a wreath Saturday where at least eight civilians died when a bus stop was shelled Thursday in Donetsk, the largest rebel-held city in eastern Ukraine.

He swiftly backtracked, however, denying that his forces were responsible for Saturday’s carnage and saying it was caused by Ukrainian error. He also said the Ukrainian defense positions around Mariupol would be destroyed but that the city itself would not be stormed.

Secretary of State John Kerry said the rebels’ new offensive “has been aided and abetted by Russia’s irresponsible and dangerous decision to resupply them in recent weeks with hundreds of new pieces of advanced weaponry.”

He urged Russia to close its international border with Ukraine and withdraw all weapons, fighters and financial backing from the separatists or face increased U.S. and international pressure.