Ukraine crisis: 'Many' pro-Russians killed and injured in Slavyansk offensive, President Turchynov claims

Two Ukrainian soldiers have also died after rebels shot down military helicopters used in 'anti-terror' operation

Many rebels have been killed, injured or arrested during the major Ukraine offensive in Slavyansk, the interim president Oleksandr Turchynov has said in a statement.

Ukrainian government forces launched an anti-terror operation to retake the rebel stronghold today.

Mr Turchynov said two Ukrainian soldiers were killed and seven wounded in Friday's clashes. The interim President said the operation was not progressing as quickly as hoped.

Earlier, pro-Russian rebels shot down two helicopters, killing two crew, after Ukrainian officials said Slavyansk was "tightly surrounded" and called on separatist leaders to release hostages and surrender.

Interior Minister Arsen Avakov said the offensive was answered with heavy artillery, with the rebels using grenade and portable anti-aircraft missile launchers to bring down planes.

By late afternoon, the Ukrainian Security Service said the army controlled half of Slavyansk - a claim that could not be immediately verified. Central Slavyansk still remained in the hands of pro-Russia gunmen, according to Associated Press journalists in the city.

Meanwhile, journalists from Sky News, CBS News and Buzzfeed who were covering the unrest in eastern Ukraine have been freed after being detained by pro-Russia insurgents.

Sky News offered no details about its crew but said they were released after being detained near Slavyansk.

CBS correspondent Clarissa Ward and her crew, and Buzzfeed Mideast correspondent Mike Giglio, were taken into custody after being stopped at a checkpoint outside the city.

Ms Ward said she and her crew were bound with tape and transported to another location, and one of her male colleagues was beaten while being held captive. Ward and Giglio said they both heard discussions among their captors about keeping the journalists as hostages.

Russia has now called for an emergency meeting of the UN Council to address the growing crisis. Moscow has asked for what will be the council's 13th meeting on the crisis, and the spokesman's office for the UN secretary-general confirmed the open meeting would be held later today.

Yesterday, Russia had vowed "catastrophic consequences" if Kiev conducted a military operation, The Voice of Russia has reported.

Vladimir Putin accused Kiev of launching a "punitive operation" in south-east Ukraine, destroying the final hope of keeping alive the Geneva agreement designed to defuse the crisis, Russian news agencies reported.

The Kremlin said Kiev's military assault "destroyed" the two-week-old agreement made in Geneva aimed at diffusing the crisis.

The European Union meanwhile has said it is watching events "with increasing concern".

Describing today's operation as "the active phase of the anti-terrorist operation", Mr Avakov said its goals were simple - "free the hostages, lay down their arms and free administrative buildings, and restore the normal functioning of the town's administration".

Additional reporting by agencies