Fighting broke out Friday around a rebel-held city in eastern Ukraine, as Ukrainian government forces began a military operation against pro-Russian separatists.
Separatist forces shot down two Ukrainian helicopters during the assault on the city of Slovyansk, killing two crew members.
A third helicopter crew member was reportedly captured by separatist forces and transferred to a local hospital.
Ukrainian Interior Minister Arsen Avakov said on his Facebook page that Ukraine demands the rebels free their hostages, lay down their weapons, vacate the buildings they have seized and restore the municipal infrastructure.
Slovyansk's pro-Russian self-declared mayor, Vyacheslav Ponomarev, told a Russian TV channel (Dozhd) that three separatist fighters and two civilians had been killed in the fighting.
In Moscow, Russian President Vladimir Putin's spokesman said the Ukrainian government's offensive launched Friday delivered a final blow to a faltering peace deal aimed at defusing the crisis in Ukraine.
Mr. Putin on Thursday demanded that Ukraine withdraw all military personnel from the troubled region near the Russian border. He made the demand in a phone conversation with German Chancellor Angela Merkel, who meets with U.S. President Barack Obama Friday in Washington.
On Thursday, Ukraine's interim President Oleksandr Turchynov signed a military conscription decree to deal with increasingly violent pro-Russian separatists who have seized buildings in about a dozen cities in Ukraine's Russian-speaking east.
Earlier this week, the Ukrainian leader said his government was "helpless" to quell the growing pro-Russian separatist movement in two eastern regions.
Mr. Turchynov also conceded that his government had lost control of its own troops in southeastern Ukraine.
Protesters control a number of key buildings in Donetsk and have declared a May 11 referendum on whether to secede from Ukraine and join Russia.
A similar vote last month led to Russia's annexation of the Crimean peninsula.