Libya's rebel leadership in Benghazi has rejected a proposal put forward by an African Union delegation to end the eight-week conflict.
The AU said Col Muammar Gaddafi had accepted the plan on Sunday. However, on Monday, his forces attacked the besieged western city of Misrata.
The rebels said they were rejecting the truce because it did not include plans for Col Gaddafi to step down.
The US, the UK and Italy have again said the Libyan leader must leave.
"The African Union initiative does not include the departure of Gaddafi and his sons from the Libyan political scene; therefore it is outdated," Mustafa Abdel Jalil from the rebels' Transitional National Council (TNC) told a news conference in Benghazi.
"The initiative speaks of reforms from within the Libyan system and that is rejected," he said.
The AU deal proposed:
- An immediate ceasefire
- The unhindered delivery of humanitarian aid
- Protection of foreign nationals
- A dialogue between the government and rebels on a political settlement
- The suspension of Nato air strikes
The presidents of South Africa, Mauritania, Mali and Congo-Brazzaville made up the AU delegation, along with the Ugandan foreign minister.
They held talks with Col Gaddafi in Tripoli on Sunday. After the meeting, South African President Jacob Zuma said the Libyan leader had accepted the AU proposal.
They then flew on to Benghazi, where they met a rough reception, with demonstrators shouting "Gaddafi out" and mobbing their vehicles, reports the BBC's Jon Leyne in the city.
International reaction to the AU proposal was lukewarm. Nato - whose air strikes are targeting pro-Gaddafi forces - said any ceasefire must be credible and verifiable.
US Secretary of State Hillary Clinton said the US wanted to see a transition which would include Col Gaddafi leaving Libya.
But Col Gaddafi's son Saif al-Islam said again it was "ridiculous" to imagine his father leaving.
A resident of Misrata who contacted the BBC said Col Gaddafi's forces had begun attacking at 0240 local time (0040 GMT), when they fired between 25 and 30 rockets.
"The situation is really hot," the resident, Abdel, said. "Gaddafi's forces are trying to enter from two sides from the east, and from inside the city centre from Tripoli Street - which is partially under control of Gaddafi's forces."
On Monday, the Libyan foreign ministry warned against any foreign intervention under cover of bringing aid to Misrata.
"Any attempt to approach Libyan territory under the cover of a humanitarian mission will be met with staunch armed resistance," the ministry said in a statement quoted by official Jana news agency.
Pro-Gaddafi forces have recently pushed back rebels - but Nato has thwarted their advance.
Nato Secretary General Anders Fogh Rasmussen called for "a complete end to violence and a complete end to all attacks against and abuses of civilians".
But he warned that military action alone would not solve the crisis in Libya, saying a solution must also offer political reforms.
The rebels have criticised Nato for not carrying out enough air strikes, as well as for a so-called "friendly fire" incident last week in which at least four people were killed.
President Zuma said Nato should end its air strikes, but the alliance said it would continue to fly sorties and would take action when it saw a threat to civilians.