WASHINGTON  With rebels in Libya calling for Western airstrikes on forces supporting Col. Muammar el-Qaddafi, Secretary of Defense Robert M. Gates warned Congress on Wednesday that even a more modest effort to establish a no-flight zone over Libya would have to begin with an attack on the country’s air defenses and would require “a big operation in a big country.”

Mr. Gates’s caution illustrates the chasm between what the rebels and some leading members of Congress are calling for and what President Obama appears willing to do in Libya. Mr. Obama and his aides have argued that it is not yet clear that the insurgents need the help  and they have warned that the use of American airpower could fuel the arguments of those in the Middle East who see a Washington conspiracy behind homegrown uprisings.

But even some members of the president’s own party sounded unconvinced on Wednesday. Senator John Kerry, the Massachusetts Democrat who is the chairman of the Foreign Relations Committee and one of the president’s chief foreign policy allies in Congress, argued that “a no-fly zone is not a long-term proposition” and warned that other nations and NATO should not be “on the sidelines” as Colonel Qaddafi’s jets begin to attack the antigovernment insurgents.

“We ought to be considering a wide range of responses, and a no-fly zone ought to be an option,” Mr. Kerry said late Wednesday. “We have a number of tools, and we should not remove any of them from the table.”