KIEV, Ukraine — Ukraine’s fragile new government accused Russia of trying to provoke a military conflict Friday by invading the Crimea region, while in Washington, D.C, President Obama warned Russia to respect Ukraine’s sovereignty, apparently in an effort to preclude a full-scale military escalation.

U.S. officials did not directly confirm a series of public statements by senior officials in the new Ukrainian government, including its acting president, Oleksandr Turchynov, that Russian troops were being deployed to Crimea, where Russia has a major naval base, in violation of the two countries’ agreements there.

Obama, however, cited “reports of military movements taken by the Russian Federation inside of Ukraine,” and he said: “Any violation of Ukrainian sovereignty would be deeply destabilizing.”

“There will be costs,” Obama said in a hastily arranged statement from the White House. He did not say what those costs might be.

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The warning came after a day in which military analysts struggled to understand a series of unusual events in Crimea, including a mobilization of armored personnel carriers with Russian markings on the roads of the region’s capital, Simferopol, and a deployment of well-armed masked gunmen at Crimea’s two main airports.

“The Russian Federation began an unvarnished aggression against our country,” Turchynov said in nationally televised remarks late Friday. “Under the guise of military exercises, they entered troops into the autonomous Republic of Crimea.”

He said Russian forces had captured the regional Parliament building, as well as the headquarters of the regional government, and that they had sought to seize other targets, including vital communications hubs, and to block unspecified Ukrainian military assets.

U.S. officials said they believed unusual helicopter movements over Crimea were evidence a military intervention was under way, but cautioned that they did not know the scale of the operation or the Russians’ motives.

Russian officials denied they had or would encroach on Ukrainian territory, and claimed any troop movements were in line with arrangements that allow Russia to station soldiers in the area.

Earlier Friday, Ukraine’s fugitive president resurfaced in Russia to deliver a condemnation of what he called a “bandit coup.”

Appearing in public for the first time since fleeing Ukraine last week, Viktor Yanukovych vowed to “keep fighting for the future of Ukraine,” while ruling out seeking Russian military help. “Any military action in this situation is unacceptable,” Yanukovych said in the southern city of Rostov-on-Don, near the border with Ukraine. Then, seeking to make a firm point, he tried — but failed — to break a pen.

At the United Nations, the Ukrainian ambassador, Yuriy Sergeyev, said that 10 Russian transport aircraft and 11 attack helicopters had arrived in Crimea illegally, and that Russian troops had taken control of two airports in Crimea. He described the gunmen posted outside the two airports as Russian armed forces as well as “unspecified” units.

“Some of them identified themselves as Russians,” Sergeyev said. He also said the Russians had captured the main air traffic control center on Crimea.

Serhiy Astakhov, a spokesman for the Ukrainian border service, said eight Russian transport planes landed in the Crimea Peninsula with unknown cargo. He said the people in the planes refused to identify themselves and waved off customs officials.

The crisis in the Crimea is the latest development in a series of fast-unfurling events that began after scores of people were killed in Kiev last week in a severe escalation of civic unrest that had been under way since late November.

Protests started after Russia pressured Yanukovych to back away from sweeping political and free-trade agreements with the European Union that he had long promised to sign, setting off an East-West confrontation reminiscent of the Cold War.

The U.S. government struggled to make sense of Friday’s rapidly evolving events. While U.S. officials said that intelligence indicated a Russian operation was under way, Obama stopped short of calling it an invasion. Part of the confusion, one official said, was that Russia routinely moves troops between military bases in Crimea.

Material from The Associated Press is included in this report.