Ukraine rebels vow new offensive as rockets kill 30

Pro-Kremlin rebels in eastern Ukraine announced a major new offensive Saturday after heavy rocket fire killed at least 30 people in the government-held port of Mariupol, sparking international calls for Moscow to stop backing the separatists.

The local mayor's office said 97 people were also wounded in the strategic city by dozens of long-distance rockets that smashed into a packed residential district early in the morning and then again shortly after noon.

"Obviously, everyone in the city is very scared," Mariupol native Eduard told AFP.

A fellow resident named Pavlo described dazed survivors helping wounded victims to climb out of the concrete rubble of Soviet-era apartment blocks and navigate streets strewn with shattered glass.

The escalating violence drew international condemnation, with United Nations Secretary-General Ban Ki-moon denouncing pro-Russian rebels for tearing up a September ceasefire and US Vice President Joe Biden saying the costs would "continue to rise on Russia" over the conflict.

Donetsk People's Republic's leader Alexander Zakharchenko had earlier said that "today, we launched an offensive against Mariupol", in quotes carried by Russia's RIA Novosti news agency.

Although he later said his forces were still "saving their strength" and had "conducted no active operations in Mariupol", he described the potential capture of the industrial port as "the best tribute possible for all our dead".

His deputy earlier denied responsibility for the civilian deaths.

But a spot inspection conducted by monitors from the Organization for Security and Co-operation in Europe (OSCE) concluded that the Grad and Uragan rocket fire came from two locations "controlled by the 'Donetsk People's Republic'".

- 'Reckless and disgraceful' -

Ukrainian Prime Minister Arseniy Yatsenyuk immediately asked the UN Security Council to censure Russia for allegedly spearheading the militants' advance on the biggest pro-Kiev city left standing in the decimated war zone.

Ukrainian President Petro Poroshenko interrupted his trip Saturday to Saudi Arabia to offer condolences after the death of King Abdullah to chair an emergency National Security and Defence Council meeting in Kiev.

"We are for peace but also accept the enemy's challenge. We will defend our motherland the way real patriots do -- until a full victory," he said in a statement.

Western leaders watched with worry as violence once again threatened to spiral out of control in what has already been one of Europe's deadliest and most diplomatically-explosive crises since the Cold War.

Both the EU and the US have imposed sanctions on Russia for fuelling the bloody nine-month conflict with troops and weapons -- an accusation Moscow has repeatedly denied.

US Secretary of State John Kerry called on Russia to "end its support for separatists immediately, close the international border with Ukraine and withdraw all weapons, fighters and financial backing".

Biden and Poroshenko in a call "expressed grave concern" about the violence and "agreed to work with international partners to ensure that the costs continue to rise on Russia for its aggressive actions against Ukraine", the White House said in a statement.

NATO Secretary General Jens Stoltenberg also urged Russia to "stop destabilising Ukraine," while German Foreign Minister Frank-Walter Steinmeier said the situation was "very dangerous".

EU foreign policy chief Federica Mogherini warned the latest escalation "would inevitably lead to a further grave deterioration of relations between the EU and Russia".

Latvia, which holds the EU's six-month rotating presidency until July, called for an emergency meeting of the bloc's foreign affairs council next week.

- Link to Crimea -

Mariupol, a city on the southeastern Sea of Azov of nearly 500,000, provides a land bridge between guerrilla-held regions to the east and the Black Sea peninsula of Crimea that Russia annexed from Ukraine last March.

More than 5,000 people have been killed and over a million displaced by the fighting, which began when deadly protests in Kiev last winter toppled Ukraine's Russian-backed president and saw the country anchor its future to the West.

A rebel assault on the port in August saw Kiev repel the attack at such heavy cost that it prompted President Poroshenko to agree to a September 5 ceasefire. That truce was, however, followed by further clashes that killed at least 1,500 people.

The separatist leader of Donetsk said on Friday he was launching an offensive aimed at seizing eastern lands still controlled by the pro-Western authorities in Kiev, a day after his men flushed out Ukrainian troops from a long-disputed airport in Donetsk.

Western diplomats linked the rebel's advance to a new infusion of Russian troops -- firmly denied by the Kremlin -- designed to expand separatist holdings before the signing of a final truce and land demarcation agreement.

Ukraine claimed on Monday that Moscow had poured nearly 1,000 more Russian soldiers and dozens of tanks into the southeast in order to secure control over factories and coal mines that could help the rebels build their own state.

Putin quickly rejected the charges and blamed Kiev for the latest surge in deaths.

"Artillery is being used, rocket launchers and aviation, and it is used indiscriminantly and over densely populated areas," Putin said on Friday.

Moscow has not yet responded to Zakharchenko's decision to discard the peace talks and go on the offensive.