President Obama warned Russia of consequences if they execute a military intervention in Ukraine. (The Washington Post)

U.S. officials said Friday that Russian troops had entered Crimea, as President Obama warned that there “will be costs for any military intervention” and vowed to stand by the Ukrainian people.

Obama said he was “deeply concerned by reports of military movements,” that “would represent a profound interference in matters that must be determined by the Ukrainian people” and would constitute a “clear violation” of international law.

As the United Nations Security Council held an emergency closed-door meeting, the United States and its allies scrambled to avoid a confrontation with echoes of the Cold War.

After a day of increasingly alarming reports of an influx of Russian troops into Crimea, and U.S. references to a “fluid” situation, Obama appeared in the White House press room to pledge that “the United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine.

“Just days after the world came to Russia for the Olympic games,” he said, “it would invite the condemnation of nations around the world.” Obama said that Vice President Biden has spoken Friday afternoon with Ukraine's new deputy prime minister “to assure him that in this difficult moment, the United States supports his government’s efforts.”

“Any violation of Ukraine’s sovereignty and territorial integrity would be deeply destabilizing, which is not in the interests of Ukraine, Russia or Europe,” Obama said.

While the president avoided confirming the Russian military movements, a senior administration official said that confirmation of an influx of troops was what led to the decision that Obama himself should speak publicly.

The U.S. officials, speaking on condition of anonymity about internal deliberations, declined to provide numbers or specific locations of Russian deployments. Ukraine’s U.N. Ambassador, Yuriy Sergeyev told the Security Council that there had been an “illegal crossing [of] the borders by Russian military transport aircraft IL-76, about 10 of them, and that 11 military attack helicopters had also violated Ukrainian air space.

The administration official said options being considered by the United States and its European partners if the Russians do not pull back included cancelling attendance at the June G8 summit to be held in Sochi, site of the recently-completed winter Olympics, and rejecting Russian overtures for deepening trade and commercial ties. The official also cited an indirect impact on the value of the ruble.

There was no overt discussion of a Western military response. Asked what Ukraine wanted the international community to do, Sergeyev told reporters after the Security Council meeting that “we want you to help us bring the truth around the world...Political support--do everything possible in insurance of preventive diplomacy. Still we have a chance to stop the negative developments...with strong voice around the world.”

The Council decided to send a mediator to Ukraine, and indicated it may hold further meetings.

Russia did not respond directly to charges of new troop movements, saying only that any action it was taking was within the confines of existing Russia-Ukraine agreements to protect Russia’s Crimean naval base at Sevastapol.

“As you know, we have an agreement with Ukraine on the presence of the Russian Black Sea Fleet,” said Vitaly Churkin, Russia’s U.N. ambassador. “We are acting within the framework of that agreement.”

Churkin, speaking to reporters, charged unnamed outsiders with provoking violence in Ukraine by recognizing the new government instead of allowing the “proper political process” to resolve the crisis there.

He said that those who had taken over control had violated an agreement reached on Feb. 21 after consultations with the foreign ministers of France, Germany and Poland that would have allowed subsequently ousted President Viktor Yanukovych to stay in office until the end of the year.

Yanukovych fled that same night after threats that opposition forces “would be storming the presidential residence unless he resigns by 10 o’clock,” Churkin said.

Asked whether Moscow had sent military forces into Crimea, Churkin said “I don’t have this specific information. I recall from history that when World War I started, some ... papers reported that they saw Cossacks at the train station. Those reports, they are not always true.”

Greg Miller contributed to this report.