Her assessment made clear the risk of possible military intervention as world powers consider broader steps to protect civilians and pressure the Libyan leader.
"A no-fly zone requires certain actions taken to protect the planes and the pilots, including bombing targets like the Libyan defense systems," Clinton said as she neared the end of a Middle East trip dominated by worries about Libya, where a rebel offensive is apparently flagging.
Clinton said discussions are going beyond specific actions toward broader authorization so countries can enforce any U.N. measures. No ground intervention is being considered, she said.
Visiting Tunisia for the first time since protesters toppled their long-time autocratic ruler, Clinton spoke about the bloodshed in neighboring Libya and cited the U.N. negotiations in New York. She said the U.S. would support U.N. actions that gain a "broad base of participation, including from Arab nations," but added that military action might be needed.
"Gadhafi must go," Clinton said, calling him "a ruthless dictator that has no conscience and will destroy anyone or anything in his way."
"If Gadhafi does not go, he will just make trouble," she said. "That is just his nature. There are some creatures that are like that."
Clinton, wrapping up the highest level U.S. visit to the Middle East since a wave of unrest began spreading through the region in December, also urged Tunisian leaders to deliver jobs and development to people hungry for economic opportunity.
"The United States stood with Tunisia during your independence and now we will stand with you as you make the transition to democracy, prosperity and a better future," Clinton said following a meeting with the new foreign minister.
She said the U.S. and Tunisia are discussing an aid package similar to one being offered to Egypt that includes investment credits, loan guarantees, insurance as well as public private partnerships in the fields of education, technology and science. She did not offer specifics.
"We need a plan for economic development, for jobs," Clinton said. "The Tunisian people deserve that."
Clinton's meeting and a news conference were moved at the last minute because of security concerns. A group of protesters had gathered outside the original venue.
During a visit to a Tunisian Red Crescent training center, Clinton said "the revolution created so many hopes, and now we have to translate those hopes into results."
The humanitarian group has received donations from the United States and taken the lead in helping refugees from Libya.
Clinton told the workers that they have "really stepped up and performed in a humanitarian way with such professionalism" even as they were presented with great demands in their own country.
She donated the first of two U.S. ambulances to the center.
The Tunisian revolution, which ousted longtime President Zine El Abidine Ben Ali in January, emboldened anti-government protesters throughout the Arab world.