Kiev (AFP) - Ukraine on Saturday sought concrete steps from Russia to back up a tenuous truce it extended with pro-Moscow rebels in the hope of calming a deadly insurgency sparked by the country's new westward course.

President Petro Poroshenko returned triumphant from Brussels on Friday having opened the way to Ukraine's eventual membership in the European Union by signing the final chapters of a landmark free trade and political association accord.

The 1,200-page tome spells out the minute details of the terms under which the splintered ex-Soviet nation will slip from the Kremlin's embrace and tie its future to European economic standards and values on human rights.

But Poroshenko had ordered his top security chiefs to meet him at the airport on landing in order to make a fateful decision about prolonging an expiring truce with rebels who have seized effective control of Ukraine's industrial east.

The 12-week insurgency has killed nearly 450 people and is viewed by both Kiev and its Western allies as Russian President Vladimir Putin's revenge for the February toppling of a leader who had ditched the very EU accord Poroshenko had signed in Brussels.

Poroshenko ultimately decided to extend the shaky ceasefire until Monday evening under the condition that Russia requires the insurgents to return border crossings to Ukrainian forces and set up a monitoring mechanism for a long-term truce.

A spokesman for Ukraine's eastern campaign told AFP that the past day of fighting saw three soldiers killed and six others wounded outside the rebel stronghold city of Slavyansk.

"Everyone knows that a bad peace is better than a good war," Defence Minister Mykhailo Koval told Ukraine's UNIAN news agency.

But "if there is not peaceful solution, we will destroy the rebels big time".

- Pressure on Putin -

Poroshenko is expected to enlist the support of German Chancellor Angela Merkel and French President Francois Hollande when he places a scheduled call to Putin on the eve of the ceasefire's expiry.

Sunday's teleconference -- the second in four days -- is primarily meant to check on any visible shift in Moscow before the European Union and Washington consider unleashing biting sanctions against Russia's financial and defence sectors the following day.

Putin has publically backed the ceasefire's extensions and promoted direct talks between Poroshenko and top rebel commanders.

But the West wants the Kremlin chief to call on the fighters to lay down their weapons and relinquish control of state buildings they had seized across a dozen eastern cities and towns.

EU leaders agreed at their Brussels summit "to reconvene at any time to adopt further significant restrictive measures if a detailed list of concrete steps are not taken by Russia and the separatists by Monday".

The United States stressed that it was also ready to act at any point.

"We have never outlined a deadline for sanctions," US State Department spokeswoman Marie Harf said in Washington.

"We can make decisions at the time of our choosing on sanctions, and we have done so and will continue to do so."

Russia's economy has already suffered from a massive outflow of capital from investors jittery about the impact of possible restrictions being imposed on the country's banks and export sectors.

Moody's on Friday cut Russia's rating outlook to "negative" due to growing "geopolitical risk".

And Russia's Economy Minister Alexei Ulyukayev admitted on Saturday that new sanctions could "seriously" impact already-stalled growth.

- Russian retaliation -

But public statements in Russia have thus far suggested that it was busy preparing an economic counter-offensive against Ukraine that would put up prohibitive barriers to its trade.

Putin's spokesman said Russia would have no choice but to impose restrictions to prevent cheap but high-quality EU goods from flooding its market after Kiev and Brussels agree to free trade.

Foreign Minister Sergei Lavrov added on Saturday that Moscow would treat Ukraine and the ex-Soviet states of Georgia of Moldova that signed their own EU deals on Friday "based on one criterium -- how (the agreements) might hurt Russian trade".

Russian and EU ministers have tentatively agreed to meet on July 11 to discuss how Moscow's concerns might be best addressed.