REPUBLIKA.CO.ID, BRUSSELS -- Ukraine's new leader on Friday sealed a landmark EU pact that drew immediate threats of retaliation by Russia in its high-stakes standoff over the ex-Soviet country's future with the West.

President Petro Poroshenko hailed the Association Agreement -- a 1,200-page document defining the political and trade terms under which Kiev will slip from the Kremlin's embrace -- as a turning point for a country that straddles a geopolitical fault line between Europe and Russia.

The deal also bursts Russian President Vladimir Putin's dream of enlisting Kiev in a Moscow-led alliance that could rival the European Union and NATO. The Kremlin immediately vowed to take "all the necessary measures" against Ukraine.

Yet the pact is just as unpopular in Russified eastern regions that mistrust the new Kiev leaders and are now witnessing a bloody separatist insurgency being waged on the streets of a dozen industrial cities and towns.

Ukraine's military said five more soldiers died overnight in attacks by rebels who have failed to honour the terms of a temporary truce agreed by their own commanders.

The European Union on Friday sealed identical agreements with Georgia and Moldova -- two former Soviet nations with equally complex relations with Russia.

Poroshenko said the deal offered Ukraine "an absolutely new perspective" and "the opportunity to modernise".

"It is a historic day, the most important day since independence," he declared.

The signing came just hours after the rebels released four unarmed monitors from the Organisation and Security and Cooperation in Europe (OSCE) they had abducted on May 26.

A top rebel leader said another four European observers and their Ukrainian translator whose release Putin has personally sought would be freed in the coming days.

The Kremlin chief denies exerting control over the fighters and is yet to address in public reports from Kiev and Washington of rocket launchers and even tanks crossing the Russian border into the conflict zone.

But Putin is also facing the threat of imminent economic sanctions should he fail to show that he is backing Poroshenko's bid to end nearly two months of fighting that have claimed more than 440 lives.

EU leaders gave Russia until Monday to change policy on Ukraine or face the prospect of tougher economic sanctions.

A statement adopted on Friday underlined the leaders' "commitment to reconvene at any time for further significant restrictive measures."

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