Sarah Rainsford in Donetsk says the scale of clashes on the ground is not yet clear. Photo: BBC

It said a pilot and serviceman had been killed, four suspected separatists held and 10 rebel checkpoints seized.

Half of the city was later declared "under control" of the Ukrainian units.

There has been no independent confirmation of the claim. Separatists at three checkpoints earlier told the BBC they were still in control there.

Russia said the use of the army by Kiev against its own people "is leading Ukraine to catastrophe".

Continue reading the main story

Fergal KeaneBBC News, Sloviansk

Describing the military operation as "punitive", the Russian foreign ministry also urged Western powers to give up their "destructive" policy on Ukraine.

The ministry earlier warned that any assaults by Ukraine's troops in the region would have "catastrophic consequences", triggering fears of an invasion by Moscow.

Separately, several foreign journalists were reportedly detained in Sloviansk and some of them were released later.

Sloviansk is a stronghold for pro-Russian separatists who are exerting increasing control in the region.

In other developments:

• Unknown attackers seize a local railway control centre near Donetsk, disrupting train movement
• Moscow has reportedly re-established contact with Russian envoy Vladimir Lukin in eastern Ukraine, after saying earlier it had been unable to get in touch with him
• Pro-Russian rebels have left the prosecutor's office and TV centre in the eastern Luhansk region

'Full combat alert'

Ukraine's anti-terror centre said in a statement that "half of the city" was under control of its units at 12:00 local time (10:00 GMT).

Ukrainian government troops reportedly seized 10 rebel checkpoints

Military helicopters were involved in the "anti-terror" operations

The rebels claimed they were still in control of some of their checkpoints

It said pro-Russian gunmen were using snipers and also mobile units dressed in riot police uniforms.

Meanwhile, the commander of Ukraine's National Guard said his units "practically cleared Sloviansk from terrorists".

Earlier, Interior Minister Arsen Avakov said the "active phase" of the operation in the Sloviansk-Kramatorsk region began at 04:30 local time.

"The terrorists opened fire with heavy weapons against Ukrainian special units.

"A real battle with professional mercenaries is going on," Mr Avakov said, adding that the separatists were using the tactics of hiding behind civilians in residential buildings.

Russia's state-run Rossiya 24 TV channel said the city was being "stormed".

It quoted Sloviansk's rebel leader Igor Strelkov as saying that the city was completely sealed off.

The fighting appeared to be concentrating on the periphery of the city, says the BBC's Sarah Rainsford, who is in the regional capital Donetsk.

In a video posted on YouTube, Sloviansk's self-declared mayor Vyacheslav Ponomariov said "our city is under attack... and we have already suffered casualties".

"I would like to appeal to all children, women and the elderly not to leave their homes."

Local residents were later quoted as saying that the situation in the city had calmed down but remained tense.

Earlier, Ukraine's acting President Olexandr Turchynov reinstated military conscription.

Mr Turchynov said his forces were "helpless" to quell the unrest in some parts of the east, saying the goal was now to prevent it from spreading.

He also said Ukraine was on "full combat alert" amid fears that Russian troops could invade.

Some 40,000 Russian troops are stationed near the Ukrainian border.

Monitors held

Eastern Ukraine has a large Russian-speaking population and was a stronghold for President Viktor Yanukovych before he was overthrown by pro-Western protesters in February.

 

 

Moscow says it reserves the right to use "all means" to protect its citizens in eastern Ukraine

Russia then annexed the Crimean peninsula - part of Ukraine but with a Russian-speaking majority - in a move that provoked international outrage.

The crisis has plunged East-West relations to their lowest point since the Cold War ended in the early 1990s.

On Thursday, German Chancellor Angela Merkel asked Russia in a phone call to President Putin to help free foreign military monitors seized by rebels in Sloviansk last week.

For his part, Mr Putin reiterated his call for Kiev to withdraw troops from the south-east to open the way for a national dialogue.