KIEV — Ukraine launched Friday what appeared to be a major assault against pro-Russian forces in the country's east.
Three deaths were reported in early fighting, including a Ukrainian pilot and a serviceman who were killed when two helicopters were shot down near the city of Slovyansk, both sides said. One pro-Russian militant was killed and another injured in the fighting, said Stella Khorosheva, a spokeswoman for the pro-Russian militants.
The intensification of clashes led Moscow to warn that any hope of implementing the Geneva agreements was now all but over.
Russian President Vladimir Putin's spokesman, Dmitry Peskov, said the offensive "effectively destroyed the last hope for the implementation of the Geneva agreements" that were intended to defuse the crisis.
The Ukrainian Security Service said its forces were fighting "highly skilled foreign military men" in Slovyansk, a city that has become a focal point for the insurgency, and that one of the choppers was shot down with a surface-to-air missile, contradicting Russian claims that the city was under the control of civilians who took up arms.
According to Ukrainian Interior Minister Arsen Avakov, the city is now fully blocked by Ukrainian forces, who have taken over roadblocks previously held by separatist rebels loyal to the Kremlin.
"Terrorists are not ashamed to shoot from the windows of apartment buildings, hiding behind the backs of peaceful citizens of Sloviansk. We are fighting hired professionals," Avakov said.
Slovyansk's self-proclaimed mayor, Vyacheslav Ponomaryov, released a video addressed to local citizens, asking women and children to stay home, and calling on men who have weapons to help defend the city.
"We were attacked. Our city is under siege. There are losses," Ponomaryov said. "I think we will defend the city. We will win."
The offensive was launched just a day after Putin warned that the Ukraine should withdraw its military from the east and south of the country.
Videos posted online show some Slovyansk citizens cheering as a Ukrainian helicopter is brought down. But some in the east believe the supporters are stuck in an old way of thinking.
"The army should have attacked the insurgents long ago," said Borys Ovcharov, from Donetsk. "Too bad it only began doing it today. The local citizens (who support the insurgents) are either paid to do it or are stuck with slavery Soviet mentality."
Ukraine's government was previously cautious about using military intervention but has decided to strike now before it loses the east further, say analysts.
"The Ukrainian government has now decided to take a more decisive approach," said Susan Stewart, deputy head of the Eastern Europe and Eurasia Research Division at the German Institute for International and Security Affairs, in Berlin.
The situation in the east could also impact the upcoming elections on May 25, added Stewart.
"Part of the thinking behind the government's approach is that if the situation becomes even less controllable in the east, then in large parts of the east the election may not be able to be held and this would call their legitimacy more strongly into question," Stewart added.
Speaking to German Chancellor Angela Merkel by phone, Putin told her that the withdrawal of troops is the "main point."
Meanwhile, Russia still has tens of thousands of troops massed along its border with Ukraine.
Jennifer Collins contributed from Berlin.