Kiev - The UN Security Council will hold a special meeting on Monday on the violence in eastern Ukraine after a rocket barrage blamed on Kremlin-backed rebels killed 30 and threatened to open up a new front in the war.
US President Barack Obama vowed to ramp up pressure on Russia after 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.
The deadly assault on the port city came a day after the insurgents pulled out of peace talks and vowed to capture new land.
The special Security Council meeting, scheduled to start at 15:00 on Monday, comes after its 15 members failed on Saturday to agree on a resolution denouncing the rocket attacks after Russia blocked the effort, according to Western diplomats.
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".
Obama said he would now look at all options -- short of military intervention -- to restrain 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 sanctions-hit Moscow.
In a call to Putin on Sunday, German Chancellor Angela Merkel urged the Russian president to "put pressure" on pro-Kremlin separatists to end the upsurge in violence, her spokesperson Steffen Seibert said.
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".
French President Francois Hollande meanwhile expressed his "very strong concern" over the Mariupol violence in talks with Poroshenko and Tusk.
'Like an earthquake'
The Kremlin flatly denies arming and funding the militants, as the West alleges. Russian Foreign Minister Sergei Lavrov on Sunday blamed the latest upswing 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 scheduled to be held in Brussels on Thursday.
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 surprise offensive.
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"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 to have launched an offensive against Mariupol, though 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 Organization for Security and Co-operation in Europe said the rocket fire came from two locations "controlled by the 'Donetsk People's Republic'".
'Infusion of Russian troops'
Mariupol, a bustling city that handles most of the southeast's vital coal and steel exports, remained calm Sunday as international monitors patrolled its muddied streets.
A rebel assault on the port in early September saw Kiev repel the attack at a heavy cost that prompted Poroshenko to pursue peace and offer the rebels three years of limited self-rule.
But the ceasefire was followed by further clashes that killed at least 1 500 people. Combat resumed in full in mid-January after a three-week lull.
Western diplomats have linked the rebel advance to a new infusion of Russian troops -- denied by the Kremlin -- designed to expand separatist territory before the signing of a final truce and land demarcation agreement.
Ukraine claimed Monday that Moscow had poured nearly 1 000 more Russian soldiers and dozens of tanks into the southeast to secure control over factories and coal mines that could help the rebels build their own state.
Monday's UN Security Council meeting will be the latest in a string of more than two dozen meetings on Ukraine since the crisis began nearly a year ago.
As a permanent member of the council, Russia has veto power and previous meetings have been marked by verbal jousting between Western powers and Moscow representatives.