Deadly clashes broke out in eastern Ukraine, officials said Saturday, after an 11th-hour U.S. effort to resolve the growing confrontation with Russia failed and Moscow shipped more troops and armor into the flash-point Crimea region.

The shootout between pro-Russian and pro-Ukrainian demonstrators took place overnight Friday in the eastern city of Kharkiv and left two people dead, acting interior minister Arsen Avakov wrote on his Facebook page Saturday. Tatiana Gruzinskaya, spokeswoman for the mayor, said the incident happened after a group of Russian separatists approached the offices being used by pro-Ukrainian activists. It was not yet clear whether the fatalities were pro-Russian or pro-Ukrainian, and activists on both sides were arrested, officials said.

The clash  is likely to add to fears that Russia will expand its intervention beyond Crimea and into eastern Ukraine. While Russia has blamed such clashes on right-wing Ukrainians and on the new government, Avakov accused Russia of inciting the violence. “Hired provocateurs from a neighboring country are staging professional provocations,” Avakov said.

With a vote planned Sunday in Crimea on breaking away from Ukraine and rejoining Russia, Secretary of State John F. Kerry warned against a “backdoor annexation” by Russia of the strategic Black Sea peninsula. But Kerry conceded that six hours of talks in London with Russia’s top diplomat neither stopped Sunday’s vote nor opened a new diplomatic path for Moscow to step back from the Cold War-tinged standoff.

The most significant U.S. and European sanctions against Russia since the collapse of the Soviet Union appeared all but certain.

“We don’t have a common vision of the situation,” Russian Foreign Minister Sergei Lavrov said after the crisis talks.

No agreements were reached, Lavrov told reporters. He stressed that Russia insists on Crimea’s right to hold the referendum. He said Russia would decide after the vote on how to respond.

When pressed about whether Russia would annex Crimea after the vote, he said, “There are no what-ifs in politics.”

Voters in Crimea will decide whether to secede from Ukraine and join Russia or remain part of Ukraine with greater autonomy.

The vote will be held under the eyes of Russian troops who effectively took control of Crimea late last month after protesters overthrew the Ukrainian government. The Crimean regional parliament has already voted to leave Ukraine, and the traditionally pro-Russian population of Crimea is expected to approve the idea as well.

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Moscow’s tightening grip on Crimea and the gathering of Russian troops along the two countries’ border have unnerved Ukrainians and left the country’s fledgling government concerned about further Russian military action. Kerry said the United States was “deeply concerned” about those deployments.

The United States and other nations have been dangling a diplomatic solution for Russia, tacitly acknowledging that the referendum would produce a pro-Russia outcome while suggesting that Russia could avert further escalation by leaving Crimea’s precise status vague.

But Kerry said Lavrov had made it clear that Russian President Vladimir Putin was not prepared to take that step. Lavrov said Russia will “respect” the results of the plebiscite, and it was clear from Kerry’s tone that the United States fears full annexation.

“We did not find common ground today on the way ahead,” said a senior U.S. official who spoke on the condition of anonymity to describe the roughly six hours of intensive discussions Friday.

In Washington, President Obama stressed the continued need for “a strong message to Russia that it should not violate the integrity and sovereignty of its neighbor.”

The European Union is expected to impose travel bans and asset freezes Monday on Russians accused of complicity in Moscow’s military incursion and the intimidation of Crimea. The E.U. on Friday identified more than 120 individuals as potential sanctions targets.

The White House announced Friday that Vice President Biden will travel to Poland and Lithuania next week to discuss Ukraine and other issues with regional leaders.

[Russia supporters in eastern Ukraine pose challenges to government]

In London, Lavrov insisted that Russia had no plans to invade Russian-speaking eastern Ukraine. But he echoed a statement issued in Moscow by the Foreign Ministry warning that Russia “reserves the right to take people under our protection.’’ Thursday night clashes between pro- and anti-Russian demonstrators in the Ukrainian city of Donetsk led to the death of one protester.

That protester was identified as Dmytro Chernyavskiy, the press secretary of the regional branch of a pro-Ukrainian ultranationalist party. Donetsk regional governor Sergei Taruta said in Kiev on Friday that the fighting was provoked by “non-Ukrainian citizens” and pro-Russian activists.

On Saturday in Kiev, Ukraine’s Foreign Minister Andriy Deshchytsa called on Russia to “stop interferring” in eastern Ukraine, adding that authorities here could handle issues of law and order in the region “ourselves.” He described threatening statements from Russia’s Foreign Ministry as an attempt to “provoke” an escalation of the tense situation in the east.

Deshchytsa called for U.N. monitors to fan out across southern and eastern Ukraine to independently assess the situation on the ground.

He also reiterated Kiev’s position that Sunday’s referendum in Crimea would be “illegal” and would not be recognised by the international community. He emphasized the new government’s desire to avoid any military conflict, and referred to the current stand off as more of a “diplomatic war.” But he added he would travel to Brussels on Monday to meet with NATO’s secretary general to discuss “military and technical cooperation.”

He said the U.N. security council was scheduled to discuss a motion on Ukraine later Saturday, but acknowledged a likely Russian veto.

Russia’s Interfax news agency reported Friday that a newly announced series of military exercises near Ukraine’s eastern border had expanded to include training missions for fighter jets and helicopters. Ukrainian military officials reported the seizure of another base in Crimea, a radar facility taken by Russian soldiers at about 3 a.m. Friday.

A Ukrainian defense ministry official said a column of about 50 armored vehicles from the Russian Federation were observed late Friday night moving from the city of Feodosia to Dzhankoi in northern Crimea. Vladislav Seleznyov said more Russian troops have moved in to occupy Ukrainian military bases that had been abandoned long before the crisis began.

In the Crimean capital of Simferopol, campaigning for Sunday’s referendum was in high gear Friday. Vans with megaphones blared Russian music in the streets around the regional parliament, where a large Russian flag already flies.

Vasilyev Maxim, an official from the Russian city of Kursk, said he drove to Crimea with thousands of Russian flags purchased with $10,000 of his own money to distribute ahead of the vote. “In 30 years, the history books will say Putin took back Crimea to rebuild our country,” he said. “And no one will remember Kerry or that Obama had anything to do with the situation.”

A new poll suggests the Crimean vote to join Russia will be overwhelming.

In a GFK poll of 600 residents taken Thursday and Friday, 70 percent said they will vote to become part of Russia, and just 11 percent said they will vote to restore Crimea’s status as part of Ukraine. If survey-takers were offered more options, they told posters, 19 percent would vote for independence. But a majority, 54 percent, would still favor becoming joining Russia.

It remains unclear how Moscow will consider Crimea’s status, said Sergei Markedonov, an associate professor of regional studies and foreign policy at the Russian State University for the Humanities.

Putin said at a recent news conference that Russia did not intend to annex Crimea, Markedonov pointed out, and the president has not made any public statements to the contrary since then. Crimea could emerge as a de facto state like Nagorno-Karabakh in the southern Caucasus, he said.

Lavrov said at his news conference that there were other precedents for Crimean secession besides the often-cited example of Kosovo. He referred to the Comoros Islands, which declared independence from France.

The talks at the sumptuous central London home of the U.S. ambassador were always a long shot to succeed.

Lavrov, a quick-witted diplomat who is often jocular when meeting with Kerry, was grim-faced as he entered the meeting. Although he speaks flawless English, he stuck to Russian at his news conference and spoke through an interpreter as he referred to the “difficult situation we are in.”

The mood may have lightened somewhat as the session continued into the afternoon, far longer than planned. Lavrov’s spokeswoman tweeted a picture of the two men kicking a soccer ball in their dress shoes as they strolled the park-like grounds of the ambassador’s residence.

Lally reported from Moscow. Karla Adam in London and Carol Morello and Pamela Constable in Simferopol contributed to this report.