President Obama on Thursday made his strongest statement to date that Libyan leader Muammar Qaddafi has “lost legitimacy” and must give up power.

“Colonel Qaddafi needs to step down from power and leave,” Mr. Obama said during a White House press conference with visiting Mexican President Felipe Calderón. That would be “good for his country, good for his people, and is the right thing to do.”

Obama also said he has instructed relevant agencies including the Pentagon to “examine a full range of options” for dealing with the Libyan crisis, including a no-fly zone over the country. But he said the priority at the moment is the developing humanitarian crisis on Libya’s eastern borders, and emphasized that any US action would be undertaken in concert with US allies and partners.

While he gave no suggestion that the international enforcement of a no-fly zone was an imminent possibility, Obama did say he wanted a list of options prepared for “protecting” Libyan civilians so that he “would be able to act potentially rapidly if the situation deteriorates rapidly.”

That statement suggested the US and other countries discussing a no-fly zone may be watching for a “trigger” of an eventual aerial campaign by Qaddafi against a rebelling population before such an extreme step would be taken. Defense Secretary Robert Gates spoke very cautiously about a no-fly zone on Wednesday, noting that such a step would essentially require the US to launch an air war on Libya to take out the country’s air-defense systems.

Some Middle East analysts have cautioned no-fly-zone enthusiasts about the potential repercussions such a move – another international, and basically Western, military intervention in a Muslim country – could have among the region’s Muslim populations.

Obama seemed to acknowledge the merits of this warning in his comments Thursday. Referring to recent events in Egypt, the president said one reason Egyptians are so “enthusiastic" about the new direction their country is taking is that it is the result of the people’s efforts and not something imposed from outside.

“They owned it,” Obama said, suggesting too much international intervention in Libya could rob the Libyans of that sentiment.

Obama’s White House statements came as the chief prosecutor at the International Criminal Court in The Hague, Luis Moreno-Ocampo, announced the ICC will proceed with a formal investigation of Qaddafi and his inner circle, including some of his sons, for “crimes against humanity” that may have been committed in the violence against Libyan civilians.

Obama said the US would continue to work with international partners on further steps to curtail Qaddafi’s actions, but it was not clear what if any of those steps would come from the United Nations Security Council in the near future.

China took the revolving one-month presidency of the Security Council this week and in comments to the press Wednesday Chinese officials dampened any expectations of an aggressive stance towards Qaddafi from the council’s chair. Li Baodong, China’s ambassador to the UN, said at a press conference at the UN in New York that the situation in Libya is “very troublesome,” but that it had to be addressed following the “principle” of “sovereignty and … territorial integrity.”

China, with one eye on its own domestic issues, has long supported a cautious approach when it comes to international intervention in another country’s internal affairs.