US and EU leaders have heaped pressure on Russian President Vladimir Putin to help his Western-backed counterpart in Ukraine end a separatist insurgency, as the clock ticks down on a shaky truce.
US President Barack Obama warned Wednesday that additional sanctions would be in store if Russia does not move swiftly to reduce tensions in restive eastern Ukraine.
In a telephone call with Italian Prime Minister Matteo Renzi, Obama also vowed he would press Russia to persuade separatist groups to abide by the fragile ceasefire agreement and "stop the flow of weapons and militants across the border", the White House said.
Kiev and Washington still accuse Putin of covertly arming the rebels in retaliation for the February ouster of a pro-Russian administration.
US Secretary of State John Kerry echoed Obama Wednesday, saying at NATO talks in Brussels that Russia must take "many concrete" steps to de-escalate the worst East-West standoff since the Cold War.
Rebel commanders had agreed Monday to a temporary ceasefire set by Ukrainian President Petro Poroshenko.
However Poroshenko on Tuesday night threatened to unleash a powerful new military campaign after the downing by a pro-Russian militia of an army helicopter, killing nine servicemen outside the rebel bastion of Slavyansk, and the loss of two troops in sporadic attacks.
The 48-year-old chocolate tycoon's ominous warning dented hopes of the sides mediating an end to 11 weeks of guerrilla warfare that has killed more than 435 people and brought the nation of 46 million to the brink of collapse.
The truce is set to end Friday after just two rounds of inconclusive talks.
Poroshenko joined German Chancellor Angela Merkel and French President Francois Hollande for an urgently-agreed conference call with Putin Wednesday, which Paris said lasted more than an hour.
The French presidency said Hollande and Merkel "encouraged" Putin and Poroshenko to "work together, especially in order to put in place a mechanism to oversee the truce".
Poroshenko's office added that the four agreed to continue the talks on Thursday in order to discuss a series of proposals made by Merkel that may be difficult for the Kremlin to accept.
- 'There is no ceasefire' -
Kiev also vowed on Wednesday to stick by its unilateral ceasefire, despite the downing of the army helicopter in the strife-torn eastern rustbelt.
Ukraine's Foreign Minister Pavlo Klimkin said he told his counterparts in Brussels on Wednesday that Kiev had thus far kept to its pledge to hold fire, despite dozens of rebel raids.
"We are committed to do our utmost to achieve the de-escalation of the situation," Klimkin said during a meeting of top diplomats from the 28 NATO member states.
But a separatist leader in the self-proclaimed Donetsk People's Republic said after a new round of peace talks with Kiev that the truce was holding in name only.
"There is no ceasefire," Oleksandr Borodai said after meeting Kiev representatives in the regional hub of Donetsk.
Putin has urged both sides to extend the truce and pushed senators to revoke his March 1 authorisation to invade his western neighbour in a bid to "protect" ethnic Russians from the nationalists now in power in Kiev.
Russia's rubber-stamp upper chamber approved Putin's request on Wednesday in a 153-1 vote.
- 'Protective measures' -
Poroshenko will introduce to parliament on Thursday constitutional changes that expand some regional powers, but stop well short of creating a federation that Putin had hoped would give the east a chance to build much closer ties with Moscow.
He will also sign the final chapters of the landmark EU pact in Brussels on Friday, despite the strong likelihood that Russia will follow up a cut in gas deliveries it imposed on June 16 with punishing new trade barriers.
"The near-term economic impact of this agreement will depend very much on how Russia responds," economists at London's Capital Economic consultancy said in a research note.
"However, the agreement, coupled with Ukraine's ($17.0-billion, 12.5-billion-euro) IMF deal, should act as an anchor for much-needed economic and political reforms which would boost growth over a medium-term horizon," it added in a report.
Russia warned Wednesday that it would take "protective measures" if Ukraine and fellow ex-Soviet states Georgia and Moldavia sign up to EU Association Accords.
Russian Foreign Minister Sergei Lavrov said such an outcome would have "negative consequences" for the regional Commonwealth of Independent States -- set up after the break-up of the Soviet Union -- and hence would require "protective measures" in line with WTO (World Trade Organisation) rules.