Rebels launched an offensive against the strategic port of Mariupol in eastern Ukraine on Saturday, prompting the European Union’s foreign policy chief to warn of a further "grave deterioration" in EU-Russian relations.

Rebels launched an offensive against the strategic port of Mariupol in eastern Ukraine on Saturday, prompting the European Union's foreign policy chief to warn of a further "grave deterioration" in EU-Russian relations.

Mariupol's city administration said the rebels had killed at least 30 people and injured 83 others in the offensive by firing rockets from long-range GRAD missile systems.

The city of 500,000 on the Sea of Azov is vital for eastern Ukraine's steel and grain exports and also straddles the coastal route from the Russian border to Crimea, the Black Sea peninsula in southern Ukraine seized by Russia last March.

President Petro Poroshenko, pledging to protect Ukrainian territory, said he would convene an emergency meeting of his country's security council on Sunday.

He said the separatists also planned to encircle Debaltseve, a town north-east of Donetsk, in the next few days, Interfax news agency quoted him as saying.

UN Secretary-General Ban Ki-moon condemned the deadly rocket attack in and denounced "provocative statements".

Ban "strongly condemns today's rocket attack on the city of Mariupol," a statement from his spokesman read.

"He notes that rockets appear to have been launched indiscriminately into civilian areas, which would constitute a violation of international humanitarian law."

The United Nations chief also condemned Friday's "unilateral withdrawal from the ceasefire by rebel leadership, and particularly their provocative statements about claiming further territory."

He said the withdrawal constituted a violation of commitments under September's Minsk accords that ordered the withdrawal of heavy weapons and a truce.

The Ban statement did not say who was responsible for the rocket fire.

Since the crisis broke out in Ukraine, the Security Council has held more than two dozen sessions on the issue, without being able to reach consensus. Moscow denies involvement with the separatists and as a permanent council member, Russia has veto power.

EU foreign policy chief Federica Mogherini condemned the Mariupol attack and urged Moscow to lean on the rebel leaders.

"I ... call openly upon Russia to use its considerable influence over separatist leaders and to stop any form of military, political or financial support," she said.

Moscow denies sending forces and weapons to east Ukraine, despite what Kiev and the West say is irrefutable proof. Last week Poroshenko said Russia had 9,000 troops stationed in his country and demanded their immediate withdrawal.

Eastern Ukraine has seen an escalation of fighting in recent days that Russian President Vladimir Putin has blamed on Kiev. The rebels have ruled out more peace talks.