Ukraine suffered its bloodiest day in nearly three months on Friday, with at least nine people killed when the army launched its first major assault on a rebel stronghold 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 in the east of the country 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.”

View Graphic Map: Russia and Ukraine are positioning their troops for war.

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 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.”

1 of 49 Full Screen Autoplay Close May 1, 2014 Thursday April 30, 2014 Tuesday April 29, 2014 Tuesday April 28, 2014 Monday April 27, 2014 Sunday Skip Ad × Ukrainian army launches first major assault against pro-Russian stronghold View Photos The Ukrainian army launched its first major assault on a rebel stronghold in the eastern Ukrainian city of Slovyansk on Friday, provoking the heaviest fighting since a pro-Russian uprising began a month ago. Two military helicopters were shot down, and at least three people were reported killed. Caption The Ukrainian army launched its first major assault on a rebel stronghold in the eastern Ukrainian city of Slovyansk on Friday, provoking the heaviest fighting since a pro-Russian uprising began a month ago. Two military helicopters were shot down, and at least three people were reported killed. May 2, 2014 A Ukrainian soldier stands in front of Pro-Russian Civilians at a checkpoint near the town of Slovyansk in eastern Ukraine. Russia has massed tens of thousands of troops along the border with Ukraine, and the Ukrainian army’s assault on Friday raised the risks of a Russian military response. Baz Ratner/Reuters Buy Photo Wait 1 second to continue.

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 Gérard 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.”

Assault on Slovyansk

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.

The rebels said three of their fighters and two civilians were killed. But Turchynov said many “criminals” were killed, wounded or taken prisoner.

Vyacheslav Ponomarev, who has been appointed mayor of Slovyansk by the insurgents, urged women, children and senior citizens to remain in their homes but asked men with guns “to render all possible assistance.”

“We will defend the city. We will win,” he said in a video message posted on the Internet.

Stella Khorosheva, a rebel spokeswoman, posted on her Facebook account: “The situation is stable on the streets, but there is a high risk of full-scale action. In short, it’s war.”

The Ukrainian Security Service said its fighters were facing “highly skilled foreign military men” in Slovyansk. It said one of the helicopters was shot down with a surface-to-air missile, which it said undercut Russia’s claims that the city is under the control of civilians who bought arms in “hunting stores.”

Swedish Foreign Minister Carl Bildt agreed. “Ukrainian helicopters shot down in Slovyansk. Some elderly ladies bought some RPGs or missiles at the local grocery store, I assume,” he posted on Twitter.

The European Union said it is watching the developments with growing concern, and NATO has said it must view Russia as an adversary in light of apparent efforts to destabilize the region following its annexation of Crimea in March. But Western leaders have made it clear they have no intention of engaging Russia militarily over Ukraine.

Putin sends envoy

Ukraine’s Interior Minister Arsen Avakov said the objective of Friday’s operation in Slovyansk was to free several hostages taken by the rebels, force the rebels to lay down their arms, release administrative buildings from their control and restore the normal functioning of the city administration.

Putin on Thursday sent a special envoy, Vladimir Lukin, to eastern Ukraine to negotiate with separatists who have taken seven international observers hostage, spokesman Dmitry Peskov told the Interfax news service. That mission was “under threat” Friday, the Russian Foreign Ministry said in a statement.

The Foreign Ministry also said that English-speaking foreigners were seen among the Ukrainian forces mounting the assault on Slovyansk on Friday, echoing its previous charges that U.S. contractors were involved in Ukraine’s response to the unrest in the east.

“The United States and the E.U. are taking on a huge responsibility in cutting of the road to a peaceful resolution of the crisis,” the Foreign Ministry said.

Nemtsova reported from Slovyansk, Ukraine. Michael Birnbaum in Moscow and Karen DeYoung and William Branigin in Washington contributed to this report.