Slavyansk, Ukraine:
Ukraine's military lost two helicopters and two servicemen on Friday in a deadly offensive launched just before dawn against pro-Russian rebels holding the flashpoint town of Slavyansk, insurgents and authorities said.
"A full-scale attack" was being mounted, said a spokeswoman for the rebels in the town.
AFP journalists reported seeing eight armoured vehicles and up to 100 troops advance to within five kilometres (three miles) of the town.
Explosions and small-arms fire were heard sporadically and helicopters were seen flying extremely low.
Ukraine's defence ministry said two military Mi-24 helicopters had been shot down and two servicemen killed.
It said the rebels were using shoulder-launched missiles to bring down the aircraft. Small-arms fire also damaged an Mi-8 helicopter but no occupants were hurt, the ministry said.
Ukrainian Interior Minister Arsen Avakov said on his official Facebook page that one of the dead was a pilot. The SBU security service said another pilot was taken prisoner by the rebels.
Avakov, who said he was in the region of Slavyansk with the defence minister and head of the national guard, underlined the sophisticated type of weaponry the pro-Russian gunmen were using.
"It's a real battle we are waging against professional mercenaries," he said, adding that the operation included the nearby town of Kramatorsk.
He said the "terrorists" were using human shields by firing from positions in big apartment blocks.
Avakov said the aim of the military operation was to force the rebels to free hostages they are holding and surrender.
- OSCE prisoners in town -
Slavyansk's insurgents have been holding seven European inspectors from the Organization for Security and Cooperation in Europe they seized a week ago.
The inspectors -- who were in Ukraine to monitor a deal aimed at ending the conflict that has largely been ignored since it was signed in Geneva on April 17 -- were being detained in the town hall. On Friday, the rebels parked one of several armoured personnel carriers they had captured last month in front of the building.
Rebels also began blocking access to the town centre and erecting new barricades.
Since mid-April, Ukraine's military has been surrounding Slavyansk, manning checkpoints at a distance of a few kilometres (miles) to prevent the militants inside receiving reinforcements.
Avakov said Friday's operation saw nine rebel roadblocks fall to the Ukrainian army.
The centre of Slavyansk -- a town with a population of 160,000 -- was itself relatively calm during the assault on the outskirts, an AFP journalist reported, but the town's church rang out its bells to alert citizens.
The rebel leadership of the town had been warning for several days that a Ukrainian military assault was imminent.
Kiev has been carrying out an operation for weeks to try to contain the rebels, but it has failed to stop them expanding their grip across east Ukraine.
Slavyansk is one of more than a dozen towns and cities pro-Russian gunmen have grabbed in the region since early last month.
The Ukrainian government and the West say the insurgency is being fomented by Moscow, which in March used the same tactics ahead of its annexation of Crimea.
Moscow denies it has any hand in the east Ukraine unrest, but warns an escalation of the Ukrainian offensive could trigger "catastrophic consequences".