“I urge Russia not to take any action that can escalate tension or create misunderstanding,” Mr. Rasmussen wrote on Twitter. “I’m concerned about developments in Crimea.” In a news conference later in the day, Mr. Rasmussen called the Crimea events “dangerous and irresponsible.”

The ministers meeting in Brussels issued similar warnings on Wednesday, saying that the alliance would “continue to support Ukrainian sovereignty and independence, territorial integrity, democratic development and the principle of inviolability of frontiers, as key factors of stability and security in Central and Eastern Europe and on the continent as a whole.”

But it remained unclear how far NATO could go in its admonishments of Russia. Ukraine is not a member of the alliance; the United States tried in 2008 to bring it in, but met with opposition from Germany and France.

The American role in the unfolding events in Ukraine, beyond sending verbal warnings to Russia and supporting financial aid for Ukraine through the International Monetary Fund, is uncertain as well. President Obama has signaled that the bar is high for any American military intervention. Though Mr. Obama is sympathetic to the pro-Western protest movement in Ukraine and to Ukraine’s new leaders, his handling of the crisis has been restrained so far.

Speaking to reporters after the NATO meeting on Ukraine, Mr. Hagel urged Russia “not to take any steps that could be misinterpreted, or lead to miscalculations during a delicate time.” He said he would be talking to General Shoigu soon by telephone.