Olga Rudenko and Luigi Serenelli

Special for USA TODAY

KIEV — Ukraine's president said Friday that "many" pro-Russian rebels were killed and wounded in a crackdown by Ukrainian troops in the Slovyansk, which had been held for days by militants demanding succession to Russia.

Russia called an emergency meeting of the U.N. Security Council because of the "serious escalation of violence" in Ukraine it said ruined any hope of upholding a truce reached in Geneva two weeks ago.

But President Oleksandr Turchynov said his government had to act against "mercenaries of foreign states, terrorists and criminals," who had refused to abide by the truce.

In Odessa, a port city on the Black Sea, 31 people died from a building fire that broke out during a clash between pro-Russia demonstrators and supporters of the Ukraine government. Odessa had largely been free of the unrest.

In Slovyansk, fighting broke out around dawn outside the city that has become the focus of the armed insurgency. Two Ukraine helicopters crashed during the operation, killing two crew members.

Ukrainian Security Service said its forces were fighting "highly skilled foreign military men," a reference to Russian troops that the United States says have infiltrated East Ukraine to direct the unrest.

By early evening, Turchynov said the army controlled all of the checkpoints around Slovyansk, but several government buildings were still held by the militants. Ukraine forces were working to prevent the unrest from spreading to other major cities like Odessa.

Interior Minister Arsen Avakov called on the insurgents to lay down their arms and release their hostages, among them five observers sent from Germany.

"We are ready to negotiate with protesters and their representatives," Avakov said. "But for terrorists and armed separatists, there is only punishment." He said "many insurgents dead, wounded and arrested."

Turchynov charged that pro-Russian insurgents in the city were hiding "behind the citizens" and "firing from apartment blocks."

"We demand that the terrorists, saboteurs, all those who took up arms against our country, to lay down their arms, surrender, release hostages and administrative buildings," he said in a statement.

President Barack Obama said Friday that the violence is making it obvious to the world that pro-Russia militants there are not peaceful protesters. Obama voiced support for the Ukrainian government and agreed that Kiev is moving to "restore order."

Obama spoke at a news conference with German Chancellor Angela Merkel, who agred that sanctions may need to be strengthened against Russia if it doesn't de-escalate the crisis.

The two Ukrainian deaths were a pilot and a serviceman killed when two helicopters were shot down by a surface-to-air missiles near Slovyansk. That proves that it is not local citizens who are directing the unrest, said Kiev.

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.

But Security Council members accused Russia of equipping and funding the pro-Russia forces that have seized government buildings in 10 eastern cities, scoffing at Moscow's claims.

"Russia … has released bands of thugs on Ukraine … and is suddenly discovering this mixture might escape its control," French ambassador Gerard Araud said.

Ukraine's foreign ministry rejected the accusations, saying Russia " did not take any steps to de-escalate the situation and fulfill the Geneva agreements."

"Russia strongly supports terrorist groups that operate in the eastern regions of Ukraine, endangering civilians, taking hostages and building the atmosphere of terror and violence," the ministry added.

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. Russia has tens of thousands of troops poised on its border with East Ukraine.

"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."

Jennifer Collins contributed from Berlin; Doug Stanglin in McLean, Va.