At least 30 people were killed and 83 others injured in the rocket attack on the port city of Mariupol, which is controlled by government troops. That's according to municipal officials loyal to Kyiv.

The Kyiv government and the pro-Russia rebels have blamed the shelling on each other, but investigators from the Organization for Security and Cooperation in Europe (OSCE), who rushed to the scene shortly afterwards, concluded that the attack had been launched from the east and northeast of the city - territory controlled by the separatists.

The rebel leader, Alexander Zakharchenko, while denying that his forces were behind the civilian deaths in Mariupol, announced that the pro-Russia separatists did intend to seize control of the city.

"Today an offensive was launched on Mariupol. This will be the best possible monument to all our dead," Zakharchenko was quoted as saying by the RIA Novosti news agency at a memorial ceremony being held in the separatist stronghold of Donetsk.

Zakharchenko had earlier rejected the ceasefire agreement which was signed in Minsk last September but has been repeatedly violated. This came just days after the foreign ministers of Germany, France, Russia, and Ukraine agreed new measures aimed at easing the fighting.

Broad international condemnation

The attack on Mariupol was met with widespread international condemnation, including from United Nations Secretary-General Ban Ki-moon. A statement released by his spokesman said Ban noted that "rockets appear to have been launched indiscriminately into civilian areas, which would constitute a violation of international humanitarian law."

"Ukraine's peace, territorial integrity and stability, intrinsically linked to that of the broader region, must be urgently restored," Ban added.

The European Union's foreign policy coordinator, Federica Mogherini, called on Russia to intercede to stop the fighting and withdraw its support for the rebels.

"This further escalation of the open armed conflict has tragic consequences for a population greatly suffering already for far too long. It would inevitably lead to a further grave deterioration of relations between the EU and Russia," a statement released by her office said.

"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," said US Secretary of State John Kerry during a visit to Zurich.

NATO Secretary General Jens Stoltenberg also urged Russia to "stop destabilizing Ukraine."

Russia has repeatedly denied supporting the rebels.

Ukrainian President Petro Poroshenko appeared defiant in light of the attack on Mariupol.

"We are for peace, but we accept the challenge of the enemy. We will protect our motherland," Poroshenko said on Saturday. He has called an emergency meeting of his security council for this Sunday.

pfd/bk (Reuters, AFP, dpa, AP)