TRIPOLI, Libya — As NATO airplanes and attack helicopters struck fresh targets in Tripoli and the oil port of Brega on Sunday, senior British and American officials said there was no way of knowing how long it might take for the rebellion against Col. Muammar el-Qaddafi — already in its fourth month, and the third month of NATO airstrikes — to drive him from power.

But Britain’s foreign secretary, William Hague, returning from a brief visit to the rebel headquarters in the eastern Libyan city of Benghazi, hinted at concern in Western capitals about what might come after the toppling of Colonel Qaddafi. Mr. Hague said he had pressed the rebel leaders to make early progress on a more detailed plan for a post-Qaddafi government that would include sharing power with some of Colonel Qaddafi’s loyalists.

In particular, Mr. Hague said, the rebels should learn from Iraq’s experience, in which a mass purge of former Saddam Hussein loyalists occurred under the American-backed program of “de-Baathification,” and shun any similar undertaking. The reference was to a policy that many analysts believe helped to propel years of insurgency in Iraq by stripping tens of thousands of officials of jobs.

According to news agency reports, crowds in Benghazi’s streets greeted Mr. Hague and Britain’s overseas aid minister, Andrew Mitchell, with shouts of “Libya free!” and “Qaddafi, go away!” as they met with leaders of the rebels’ Transitional National Council, headed by Mustafa Abdul Jalil, who was justice minister in Colonel Qaddafi’s government until the rebellion began in February. Back in London, Mr. Hague described the rebel leaders as “genuine believers in democracy and the rule of law,” but said that they should make more detailed post-Qaddafi plans.