Obama ramps up pressure on Russia after deadly Ukraine blitz

Kiev — US President Barack Obama vowed Sunday to ramp up pressure on Russia after a rocket barrage blamed on Kremlin-backed Ukrainian rebels killed 30 and threatened to open new front in the war.

Saturday's assault on Mariupol — the main city standing between separatist territory near the Russian border and the Black Sea peninsula of Crimea that Moscow annexed in March — came a day after the insurgents pulled out of peace talks and vowed to capture new land.

Ukrainian President Petro Poroshenko told an emergency security meeting that “Kiev had intercepted calls proving the attack was masterminded by terrorists who receive support in Russia".

And Obama said he would now look at all options — short of military intervention — aimed at restraining Russian President Vladimir Putin's alleged proxy war aimed at stripping Ukraine's pro-Western leaders of their vital eastern industrial base.

He pledged to "ratchet up the pressure on Russia" and signalled that he took a dim view of some EU members' desire to revive their ailing economies by restoring full financial and trade ties with Moscow.

"If Mr Putin and if Russia are hell-bent on engaging in military conflicts, their military is more powerful than Ukraine's," Obama said during a visit to India.

"The question is going to be whether they continue to pursue a path that not only is bad for the people of Ukraine, but is... bad for the people of Russia."

New European Council President Donald Tusk — a former Polish prime minister who had long been suspicious of Putin — also called the Mariupol attack evidence that "appeasement encourages the aggressor to greater acts of violence”.

"Time to step up our policy based on cold facts, not illusions," Tusk tweeted.

The Kremlin flatly denies arming and funding the militants. Russian Foreign Minister Sergei Lavrov on Sunday blamed the latest upsurge in violence on "constant shelling" by Kiev's troops.

"Lavrov pointed out that an escalation of the situation is a result of Ukrainian troops crudely violating the Minsk agreements by constantly shelling residential settlements," the foreign ministry said after Russia's top diplomat spoke to US Secretary of State John Kerry by phone.

Poroshenko told his top generals that he had asked the European Union to tighten their sanctions on Russia at a special session of foreign ministers on Monday.

The Western-backed leader — looking tired after cutting short his attendance at the burial of the late Saudi king — also insisted that the attack would not provoke Kiev into ordering a tough military response.

"Ukraine remains a firm proponent of a peaceful solution," he told a televised meeting of his National Security and Defence Council.

Regional police said 95 people were also wounded by dozens of long-distance rockets that smashed into a packed residential district and a market in Mariupol on Saturday.

"I was alone at my house when the shelling began," a boy named Viktor Zarubin told AFP outside a Mariupol church that was having a mass for the victims of the surpirse offensive.

"I lay down on the ground and I crawled into the basement. It was like an earthquake,' the 15-year-old said.

The self-proclaimed Donetsk People's Republic leader Alexander Zakharchenko claimed Saturday that "today we launched an offensive against Mariupol".

He later distanced himself from the rocket fire and denied ordering an actual invasion of the industrial port on the Sea of Azov.

But the Organisation for Security and Cooperation in Europe said the Grad and Uragan rocket fire came from two locations "controlled by the 'Donetsk People's Republic'".

Mariupol remained calm on Sunday as international monitors patrolled its muddied streets.

The bustling city is home to two of Ukraine's largest smelters and most of the southeast's vital coal and steel exports go through its docks.