Militants shot down a military helicopter near Sloviansk city in eastern Ukraine, BBC reported Monday citing a Ukrainian defence ministry statement.
It was the third such incident of a Ukrainian military helicopter being downed over the city in recent days.
The crew of the aircraft, however, survived largely because the helicopter crashed into a river, it added.
Meanwhile, pro-Russian militants just outside Sloviansk retreated amid attacks by Ukrainian troops, reports said.
Government forces took control of a TV tower in the suburbs and rebels pulled back deeper into the city, the Russian media reported. Earlier reports told of heavy gunfire, apparently closer to the centre than in recent days.
However, a BBC team which reached the centre of Sloviansk, reported a lull in the fighting between about 800 pro-Russian militants and Ukrainian troops.
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Residents had reported heavy gunfire Monday morning.
Four Ukrainian soldiers were killed and 30 injured during the fighting in Sloviansk, the interior ministry said.
Ukrainian troops are currently carrying out what the Kiev government calls "anti-terror" operations in the east to wrest back control of these areas.
Ukraine's President Viktor Yanukovych was overthrown by pro-Western protesters in February. Russia later annexed the Crimean peninsula -- part of Ukraine but with a Russian-speaking majority. The move sparked off international outrage.
Separatist actions subsequently spread to eastern Ukraine, where Moscow is accused of backing pro-Russians who have seized official buildings in a dozen or more cities.
Despite the latest violence, the country plans to hold a presidential election May 25.