By David M. Herszenhorn, Mark Landler and Alison Smale

New York Times News Service

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 President Barack Obama issued a stern warning to the Kremlin about respecting 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.

The pointed 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 Friday evening. “Under the guise of military exercises, they entered troops into the autonomous Republic of Crimea.”

He said that Russian forces had captured the regional parliament, 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 that the unusual helicopter movements over Crimea were evidence that a military intervention was underway, but cautioned that they did not know the scale of the operation or the Russians’ motives.

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

A new government

Still, developments in Ukraine sent Ukraine’s interim government, appointed just the day before, deep into crisis mode as it confronted the prospect of an armed effort to split off Crimea, an autonomous region with close historic ties to Russia, from the Ukrainian mainland.

Analysts said the increase in the Russian presence in the area had parallels to steps President Vladimir Putin took before beginning a war with Georgia in 2008 over the largely ethnic Russian regions of Abkhazia and South Ossetia. But there was little to indicate if Putin intended to escalate the challenge to Ukraine beyond the so-far nonviolent provocation of the mostly pro-Russian population in the region.

Turchynov, the acting president, also made comparisons to Georgia.

“They are provoking us into military conflict,” Turchynov said. “They began annexation of territory.”

In his address, Turchynov added: “I personally appeal to President Putin, demanding that he immediately stop the provocation and withdraw troops.”

The crisis in the Crimea, along the Black Sea, 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 underway since late November.

Protests started after Russia pressured then President Viktor 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.

After the recent killings, Yanukovych reached a tentative truce with opposition leaders in talks brokered by the foreign ministers of France, Germany and Poland, but within 24 hours he fled the capital, and an overwhelming majority of lawmakers voted to strip him of power, saying he had abandoned his position.

On Friday, a week later, Yanukovych resurfaced for a news conference in Rostov-on-Don, Russia, in which he said he was still the legitimate president and urged Russia not to intervene militarily in the Crimea.

U.S. response

Obama’s warning suggested a deepening uncertainty among U.S. officials about Putin’s intentions in the region despite a series of high-level contacts in recent days, including a telephone call between the two presidents one week ago. Yanukovych was an ally of Russia and his toppling has left the Kremlin grappling for a response.

Washington, meanwhile, has struggled to make sense of the rapidly evolving events in Crimea. While U.S. officials said that intelligence indicated a Russian operation was underway, 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.

Another U.S. official said that intelligence reports from the region were “all over the place,” but that the administration believed that Russia had moved some of its forces into Ukraine, while some of the movement, officials said, seemed to be an increase in protective measures around Russian military installations.

Although he threatened an unspecified “cost” to Russia, Obama appeared to have limited options to respond to an intervention. Officials said he could cancel his participation in a Group of Eight meeting in Sochi, Russia, next June. The administration could also shut down talks on a potential trade agreement. Russia sent a delegation to Washington this week to explore closer trade and commercial ties.

Crimea, a multi-ethnic region that was granted a large degree of autonomy in 1992 after Ukraine gained independence from the Soviet Union, has long been a source of tension with Russia, and is the headquarters of some of Russia’s most important military installations, including the headquarters of its Black Sea naval fleet.