"This is not an outcome the U.S. or any of our partners sought," Obama said from Brazil, where he is starting a five-day visit to Latin America. "But we cannot stand idly by when a tyrant tells his people there will be no mercy."
American and British warships hurled 110 Tomahawk missiles at Gadhafi's air-defense systems on Saturday, striking roughly 20 targets around Tripoli and the western city of Misurata, according to Vice Adm. William Gortney, director of the Joint Staff.
The attacks were aimed at longer-range air defense missiles and early warning radar sites.
The United States cranked up the pressure on Gadhafi following reports that he was continuing violent attacks against the country's opposition forces despite international demands for a cease-fire.
Obama said military intervention was not his first choice and reiterated that no ground troops would be sent to Libya.
"I am deeply aware of the risks of any military action, no matter what limits we place on it," he said. "I want the American people to know that the use of force is not our first choice, and it's not a choice that I make lightly."
The president and other U.S. officials continue to openly reject taking a leadership position in international efforts against Gadhafi.
"We did not lead this," Secretary of State Hillary Clinton said outside a summit meeting in Paris. "We did not engage in unilateral actions in any way, but we strongly support the international community taking action against governments and leaders who behave as Gadhafi is unfortunately doing so now."
The operation is being led by France and Britain, with significant U.S. support, according to the Pentagon. The U.S. role will largely be limited to knocking out Libyan air defenses, which will enable American allies to enforce a no-fly zone over the country. In response to the rising tide of international pressure, Gadhafi has written letters to Obama and other leaders saying that they would pay for intervening in Libya.
"You will regret it if you take a step toward intervening in our internal affairs," he wrote in one letter, read to the news media by a spokesman. "This is injustice, it is clear aggression, and it is uncalculated risk for its consequences on the Mediterranean and Europe."
In a letter to Obama, Gadhafi claimed that the Libyan rebels fighting for his ouster are aligned with Al Qaeda.
"We are confronting Al Qaeda in the Islamic Maghreb, nothing more," he wrote. "What would you do if you found them controlling American cities with the power of weapons? Tell me how would you behave so that
I could follow your example?"
The North Atlantic Treaty Organization will take over enforcement of the no-fly zone and arms enforcement by mid-week, according to NATO officials.
Obama will be returning from his trip to Latin America on Wednesday, as previously planned, despite the military advances abroad.