"I don't want us hamstrung," Obama said. He cited the possibility of a humanitarian crisis, or "a situation in which defenseless civilians were finding themselves trapped and in great danger," or "a stalemate that over time could be bloody" if Libyan leader Moammar Gaddafi continues to resist international demands that he step down.

Gaddafi "has lost legitimacy to lead, and he must leave," the president said.

But in his first public statement on Libya since the outbreak of widespread armed conflict between opposition forces and those loyal to Gaddafi, Obama expressed several notes of caution, stressing that the United States must act only "in consultation . . . with the international community."

"The region will be watching carefully to make sure we're on the right side of history," Obama said at a White House news conference with visiting Mexican President Felipe Calderon. As with Egypt and Tunisia, he said, U.S. interests were best served if the United States was not seen as engineering or imposing a particular outcome.