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Kyiv, Ukraine (CNN) — Residents of the flashpoint city of Slavyansk were warned to stay home and avoid windows Friday as Ukrainian security forces launched an “anti-terrorist operation” aimed at dislodging separatist militants.

The operation, also targeting the town of Kramatorsk, appears to be the most significant yet by the Ukrainian military against pro-Russia militia groups who’ve taken effective control of swaths of eastern Ukraine.

Conflicting reports are emerging, but it appears the operation has already claimed its first casualties.

Two Mi24 helicopters have been taken down with mobile air defense systems, killing two military officers and injuring others, according to the Ukrainian Defense Ministry website.

Another army helicopter, an Mi8, was damaged but no one was hurt, the statement said.

Interior Minister Arsen Avakov said a pilot had been killed and “some others” were injured. Ukraine’s Security Service, the SBU, said one helicopter had been taken down, leading to the death of one pilot while another was taken hostage.

Russian state news agency RIA Novosti earlier reported that one Russian separatist was killed and another wounded in Slavyansk.

“The terrorists opened fire at Ukrainian units with some heavy guns, including grenade launchers and portable air defense systems,” Avakov said, in a post on his official Facebook page.

Nine checkpoints that were under control of pro-Russian separatists within the city of Slavyansk have been taken back by Ukrainian forces, who now encircle the town, according to the minister’s post.

The operation is being conducted by the Interior Ministry, the National Guard and the army, he said.

Avakov urged residents not to go outside and to be careful at windows while the operation continues. The separatists are hiding among the civilian population and “shoot from the windows of residential apartments,” he said, aware that the Ukrainian forces have been told not to fire toward homes.

The SBU also accused separatist leaders of ordering activists to use residents as a human shield within the city and at checkpoints.

The service said the downing of a military helicopter indicated that those shooting were “highly professional foreign military, rather than peaceful residents with hunting guns, as the Russian leadership says.”

A CNN team north of Slavyansk saw Ukrainian military units on the road, and heard the sound of two explosions that may have been rocket propelled grenades.

Pro-Russian activists in the city control key buildings and have held a team of Western military observers from the Organization for Security and Cooperation in Europe captive for the past week.

What the Ukrainian authorities want from the separatists has not changed, Avakov said — release the hostages, turn in weapons, vacate seized administrative buildings and allow the normal functioning of the city.

As part of efforts to isolate the town, trains will be blocked and road traffic is being kept to a minimum, he said.

Previous phases of the “anti-terror operation” by the Ukrainian forces have not resulted in any significant gains.

Donetsk clashes

On Thursday, pro-Russian activists and Ukrainian riot police clashed at the prosecutor’s office in the eastern city of Donetsk as simmering tensions escalated into violence.

At least one police officer was injured as the separatists seized control, the Ukrainian Interior Ministry said, adding that shots were fired and small grenades and stones were used in the attack.

Police fired tear gas and stun grenades in an effort to disperse the activists, who were armed with clubs, batons and shields.

The regional health authority said 26 people were injured, four of them with gunshot wounds.

Earlier in the day, crowds marched through Donetsk, demanding greater autonomy for the restive eastern region.

Many in the region view the interim government in Kyiv as a “junta” that seized power thanks to backing from ultranationalist groups, and they are angered by its actions.

Separatist leaders want to hold a referendum on May 11 on Ukraine becoming a federal state.

Eastern Ukraine was a heartland of support for pro-Moscow President Viktor Yanukovych, ousted in February after months of protests by people upset that he had turned away from Europe in favor of Moscow.

The crisis has sparked deep divisions in Ukraine. Many also want to see the country remain united, but unhappiness about government corruption and ineffectiveness runs deep.

The interim government has said it’ll look at constitutional reforms ahead of national elections due on May 25.

IMF approves $17.1 billion bailout

Acting Ukrainian President Oleksandr Turchynov acknowledged this week that the central government has effectively lost control of the country’s Donetsk and Luhansk regions to the pro-Russian separatists.

He signed a decree introducing military conscription Thursday in a bid to beef up Ukraine’s military, citing “real and potential threats to Ukraine.”

Besides the threat from pro-Russian separatists, NATO estimates that Russia has some 40,000 troops massed near Ukraine’s border.

In a key sign of international support for the Kyiv authorities, the International Monetary Fund approved a $17.1 billion bailout for Ukraine on Thursday.

The bailout, which is conditional on structural reforms, should also unlock $15 billion in additional international funding, IMF chief Christine Lagarde told CNN’s Christiane Amanpour.

Lagarde also noted that “clearly there have been consequences” for the Russian economy as a result of its intervention in Ukraine.

Russia annexed Ukraine’s southeastern Crimea region in March following a controversial referendum. Its actions have prompted fears it may seek also to intervene directly in eastern Ukraine, which has a large Russian-speaking population.

CNN’s Nick Paton Walsh reported from near Slavyansk and Victoria Butenko from Kyiv, while Laura Smith-Spark wrote and reported from London. CNN’s Arwa Damon in Donetsk and Claudia Rebaza in Kyiv contributed to this report.