Suspicion over the recent murder of the opposition forces commander Abdel Fattah Younes has fallen on a group of Islamists from the town of Derna, in eastern Libya.
Muammar Gaddafi's government has described the town as a "hotbed of religious fundamentalism" with widespread ties to al-Qaeda, a claim that that the town's residents adamantly refute.
Saif al-Islam, Gaddafi's son, has told the New York Times that he is forging an alliance with Islamist fighters against their liberal allies, in what appears to be a bid to divide the Libyan opposition groups.
The newspaper interviewed Ali Sallabi, a senior Islamist opposition leader, who confirmed he had been in contact with Gaddafi's son.
But he denied forging an alliance with the ruling family, pledged his continued support for the opposition and denied a split with the liberal wing of the six-month-old uprising.
"Saif al-Islam's statement is baseless. It's a lie that seeks to create a crack in the national accord," Sallabi said.
"Our dialogue with them [Gaddafi's family] is always based on three points: Gaddafi and his sons must leave Libya, the capital [Tripoli] must be protected from destruction and the blood of Libyans must be spared. There is no doubt about these constants.
"We support pluralism and justice. Libyans have the right to build a democratic state and political parties."
Sallabi said relations between the Islamists and secularists in Libya are "strong".
Meanwhile, on the ground, the opposition scored a significant victory on Thursday, bringing a ship with a seized cargo of government-owned fuel into their port.
The docking in Benghaz, the opposition stronghold, of the Cartagena, a tanker carrying at least 30,000 tonnes of petrol, is expected to boost the uprising, which has won broad international military and diplomatic backing but is struggling to remove Gaddafi.
Gaddafi has so far remained in control of the capital, Tripoli, despite severe fuel shortages and opposition advances on three fronts, backed since March by Western air raids.