Pro-Russian rebels have shot down a government helicopter in eastern Ukraine, according to a military spokesman.
The Ukrainian national guard has claimed that separatists brought down an MI-8 shortly after take-off near Slovyansk, with all nine members of the crew feared dead.
If confirmed, the incident would mark the end of the ceasefire declared by Ukrainian president Petro Poroshenko last week. Leaders of the pro-Russian insurgency in eastern Ukraine had previously announced that they would respect the truce.
The development came after Russian president Vladimir Putin effectively put an end to his country's military threat to Ukraine, asking the upper house of the Russian parliament, the Federation Council, to revoke the right to use force in the neighbouring country.
The Federation Council authorised Putin to use force in Ukraine on 1 March, after Kiev's pro-Russian President Viktor Yanukovich was ousted following months of street protests. Russia has since annexed Ukraine's southern Crimea peninsula.
Putin had previously announced his support for a peace plan drafted by Ukraine as long as it includes "practical action" to start talks. But he said that the truce declared by Poroshenko must not be used as an "ultimatum".