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 reported captured by separatist forces and transferred to a local hospital.
Slovyansk's pro-Russian self-declared mayor, Vyacheslav Ponomarev, told a Russian TV channel (Dozhd) that three separatist fighters and two civilians were killed in the fighting.
Ukraine's Acting President Oleksandr Turchynov said Friday that the rebels had suffered heavy losses, including many killed and wounded.
In the southern Ukrainian city of Odessa, at least four people were killed Friday in fighting between pro-Russian demonstrators and supporters of Ukrainian unity.
In Washington, President Barack Obama said at a joint news conference with German Chancellor Angela Merkel that if Russia's leadership continues to destabilize eastern Ukraine, the U.S. and European Union will move quickly to impose additional steps -- including both diplomatic and economic sanctions.
Meanwhile, Russia on Friday called for a United Nations Security Council meeting over the Ukrainian security operation in eastern Ukraine.
Earlier in the day, Russian President Vladimir Putin's spokesman said the Ukrainian government offensive had 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, President 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.