DONETSK, Ukraine -- Ukraine suffered its bloodiest day in three months on Friday, with at least nine people killed when the army launched its first major assault on a rebel stronghold in the east of the country and 34 killed in clashes between pro-Ukrainian and pro-Russian mobs in the Black Sea port city of Odessa.

The Ukrainian army attacked Slovyansk at dawn, provoking the heaviest military fighting since a pro-Russian uprising began a month ago. The army took control of the major checkpoints outside the city but was unable to force its way into the center, and two of its helicopters were shot down.

The biggest loss of lives came in Odessa. A pro-Ukrainian rally attended by thousands of soccer fans before a game Friday night was attacked by pro-Russian separatists. The two sides fought running battles through the city in the afternoon, throwing stones at each other and erecting barricades. At least three people were killed.

Friday evening, a pro-Ukrainian mob attacked a camp where the pro-Russian supporters had pitched tents, forcing them to flee to a nearby government building, a witness said. The mob then threw gasoline bombs into the building. Police said 31 people were killed when they choked on smoke or jumped out of windows.

Asked who had thrown the molotov cocktails, pro-Ukrainian activist Diana Berg said, "Our people -- but now they are helping them to escape the building."

It amounted to the deadliest day in the Ukrainian crisis since February, when scores of people were killed, many by snipers, in protests against then-President Viktor Yanukovych.

Ukraine's acting president, Oleksandr Turchynov, said Friday that the military assault on the eastern city of Slovyansk was intended to protect civilians from "mercenaries of foreign states, terrorists and criminals who are taking hostages, killing and torturing people, and threatening the territorial integrity and stability of Ukraine."

A spokesman for Russian President Vladimir Putin called the offensive a "criminal" act and said it had "effectively destroyed the last hope for the implementation of the Geneva agreements" reached April 17 that were intended to defuse the crisis. Under the accord, signed by Ukraine, Russia, the United States and the European Union, separatists were supposed to lay down their arms and vacate government buildings they have occupied across eastern Ukraine.

Ukraine accuses Russia of financing and arming the separatists, who have vowed to hold a referendum on independence on May 11. Russia denies the charge.

In Washington, President Barack Obama expressed strong support Friday for the Ukrainian offensive and said the United States and Europe "are united in our determination to impose costs on Russia for its actions" in destabilizing eastern Ukraine.

Russia has massed tens of thousands of troops along the border with Ukraine, and Friday's developments raised the risks of a Russian military response. Russian officials have said they would intervene in Ukraine to protect ethnic Russians, and the Kremlin said Friday that Putin was "closely following developments."

At the U.N. Security Council, Russia and the United States again traded accusations and insults. Using Cold War language, Russia's ambassador to the United Nations, Vitaly Churkin, said that if the "criminal misadventures of the Kiev clique are not stopped, . . . catastrophic consequences cannot be avoided."

Churkin charged that "English-speaking foreigners" had been overheard in radio communications during Ukraine's current "punitive operations" against the separatists.

The United States, France and Britain were scathing in their responses.

"A pyromaniac fireman situation is what we have here," French U.N. Ambassador Gerard Araud said. Russia is "screaming in order to make us forget that this path was set long ago, and it's no longer possible to go backwards."

Ukrainian troops attacked the rebel stronghold of Slovyansk at dawn, meeting heavy resistance from pro-Russian separatists. After three or four hours of clashes, the army had taken control of checkpoints on the main roads leading to the city, but the center of Slovyansk remained in rebel hands. The fighting subsided by the afternoon but resumed in the evening.

Two Ukrainian Mi-24 attack helicopters were shot down and two crew members killed, while seven soldiers were wounded in the offensive, the government said. One helicopter pilot was captured by the rebels and transferred to a hospital, medics told local news media.

Ukraine's Defense Ministry said a third helicopter, an Mi-8 transport reportedly carrying medics, was also hit and a service member wounded.