The Ukrainian military says one of its helicopters has been shot down by pro-Russian rebels in the east, killing all nine people on board.
It says the Mi-8 helicopter was hit by a rocket shortly after take-off outside the rebel-held city of Sloviansk.
It comes a day after the rebels vowed to observe a ceasefire until Friday, in response to a government peace plan.
But Ukraine's president said he may end it due to "constant violation by rebels who are controlled from abroad".
A statement from Petro Poroshenko's office said "the head of state does not exclude that the ceasefire regime may be revoked ahead of schedule".
In a further sign that the truce might be unravelling, Alexander Borodai, leader of the self-styled Donetsk People's Republic which is defying the Ukrainian government, said that in his view there was no point keeping to the ceasefire.
"I say officially now that there has been no ceasefire and, judging by everything, there will not be any," he told Russian television .
"In general, all that is left to us is to continue fighting," he added. On Monday, Mr Borodai said his forces would observe a ceasefire until Friday morning.
Analysis: David Stern, BBC News, Kiev
One would suspect that the Ukrainian government's truce in the east is now a dead letter. President Petro Poroshenko's plans to respond to the attack are still unclear, but public outrage alone would seem to demand some sort of military retaliation.
And if the Ukrainian government uses force, then very likely the Ukrainian insurgents and their Russian comrades-in-arms will answer in kind. An escalation seems inevitable.
At this point, it is close to impossible to determine why the militants decided to carry out such a provocative act, just one day after they declared a ceasefire. Maybe this was some rogue element. Maybe the insurgents were never serious. Maybe Moscow told them to do it.
Whatever the reason, the hopes of just 24 hours ago, that Ukraine's east could finally see peace, if only temporarily, ring especially hollow.
Russian President Vladimir Putin said the truce should be extended to try to hold "substantive talks" between the Ukrainian government and the separatists.
The rebels have not commented publicly on the downing of the helicopter.
Before Tuesday, the separatists - who continue to hold towns in the regions of Donetsk and Luhansk - had shot down at least two Ukrainian army helicopters and a plane.
Earlier in the day, President Putin asked the Russian parliament to revoke the right of military intervention in Ukraine.
The move was aimed at "normalising the situation" in the eastern regions of Ukraine, Mr Putin's press secretary said.
The parliament authorised Mr Putin to use force in Ukraine on 1 March.
Mr Poroshenko said Mr Putin's latest move was a "first practical step" towards settling the crisis in the east. It came after Russia had officially supported Ukraine's peace plan, which included the week-long ceasefire.
But speaking later during a visit to Austria, Mr Putin stressed that revoking the right on using force did not mean that Russia would stop protecting "ethnic Russians in Ukraine... who consider themselves part of the broad Russian world".
Since March, Moscow has annexed Ukraine's southern Crimea peninsula - a move condemned by Ukraine and Western leaders.
The takeover achieved with the help of troops without insignia. Despite initial denials, Mr Putin later admitted that they were Russian armed forces.
However, Russia denies accusations by Ukraine that Russian troops are helping and arming the separatists in eastern Ukraine.