An African proposal to end Libya’s more than month-old conflict unraveled Monday in the opposition’s de facto capital, where rebel leaders insisted that they would not make a deal unless leader Moammar Gaddafi agreed to leave office immediately.

The rejection came less than 24 hours after South African President Jacob Zuma, who led an African Union delegation to Libya, said Gaddafi had accepted the “road map” to peace. The plan called for an immediate cease-fire, safe passage for humanitarian aid, the protection of foreign nationals and the start of discussions about reform between the government and the opposition.

“The African Union initiative does not include the departure of Gaddafi and his sons from the Libyan political scene,” opposition leader Mustafa Abdel Jalil told reporters after meetings in Benghazi between the A.U. delegation and rebel leaders. “Any future proposal that does not include this, we cannot accept,” he said.

The Obama administration echoed the rebels’ insistence on Gaddafi’s departure as a precondition for any political settlement. “It’s a non-negotiable demand,” State Department spokesman Mark Toner told reporters at a Monday news briefing. “We believe he needs to depart power.”

Toner said the administration’s chief envoy to the Libyan opposition, Chris Stevens, was continuing to meet with rebel leaders in Benghazi to discuss a future democratic transition and how the United States could help. “We continue to believe that through diplomatic pressure we can end” the fighting, Toner said.

Gaddafi’s forces continued to attack the isolated port city of Misurata, about 130 miles east of Tripoli.

NATO said Monday that its forces had hit 11 tanks and five other vehicles belonging to Gaddafi’s forces in strikes near Misurata and the eastern city of Ajdabiya.

“We know we’re having an effect — his forces are showing signs of confusion,” Lt. Gen. Charles Bouchard, the commander of NATO’s Libya operation, said in Brussels. “But the pressure will remain and our forces are determined to protect civilians from these attacks.”

Members of the rebels’ Transitional National Council — largely made up of doctors, lawyers, intellectuals, defectors and former exiles — were deeply skeptical of the African Union, which they see as packed with Gaddafi allies, and the visiting delegation received a cool reception.

As meetings went on inside Benghazi’s Tibesty Hotel, thousands of protesters gathered outside to express their dissatisfaction with the plan, chanting denunciations of Gaddafi that grew louder throughout the day.

“African Union, do you know how many people Gaddafi killed??” a sign held by one protester said.

“First Gaddafi leaves, then we negotiate,” a sign draped above the front door of the hotel said.

Rebel leaders said they don’t believe Gaddafi would adhere to a cease-fire, a pledge he has made and breached several times in recent weeks. They said the African delegation was staged to buy time while Gaddafi’s forces regroup and press their attacks against the opposition.

“Gaddafi was never honest in his conduct with anyone,” said Gen. Khalifa Hiftar, the commander of the rebel army’s ground troops.

The A.U. delegation included the presidents of the Republic of the Congo, Mali and Mauritania and the foreign minister of Uganda. Zuma was with the group in Tripoli on Sunday to meet with Gaddafi, but he returned to South Africa when the others went on to Benghazi, and issued a statement from there calling the trip a success.

Staff writer Joby Warrick in Washington contributed to this report.