"We have contacts with people from the inner circle of Gadaffi," said the chairman of the rebel National Transitional Council (NTC). "All evidence (shows) that the end is very near, with God's grace."
Abdel Jalil was speaking to reporters as a flurry of rumours suggested that Gadaffi was preparing to flee Libya.
"I expect a catastrophic end for him and his inner circle, and I expect that he will a create a situation within Tripoli. I hope my expectation is wrong," Abdel Jalil said.
"That would be a good thing that will end the bloodshed and help us avoid material costs. But I do not expect that he will do that," Abdel Jalil added.
Earlier, rebels claimed to have captured the strategic eastern oil hub of Brega, a day after saying they had seized two other key towns.
In another blow to Kadhafi, the rebels also said former premier Abdessalam Jalloud, who fell out of favour with the Libyan strongman in the mid-1990s but remains a highly popular figure, had defected and joined their ranks.
Jalloud "has gone to Benghazi yesterday night (Friday)," rebel spokesman Juma Ibrahim told reporters Saturday. "I don't know who he met there. He left by car," he added.
Overnight, rebel military commander Colonel Ahmed Omar Bani told AFP the ex-premier had managed to flee Tripoli and "has joined the rebels." Another source said his family accompanied him and they stopped first in Zintan.
His defection comes amid amid rumours that the Libyan strongman himself was preparing to flee as rebels appear to be closing in on the capital.
Jalloud was among the officers who grabbed power with Gadhafi in 1969 and was long considered the regime's second-in-command before being gradually sidelined in the 1990s.
Prime minister during the 1970s, he retired from politics following his dispute with Kadhafi and lived under hour arrest.
Libya's Awalam television channel quoted the former premier on its news ticker as saying: "Gaddafi's regime is finished."
On the ground, a top-ranking rebel official said of Brega, "the industrial zone is under our control; all Brega is now under our control."
On Friday they claimed the western refinery town of Zawiyah to be free, the last major barrier as they try to advance on Tripoli from the west.
The refinery is the only source of fuel to the capital, and could leave it without critical supplies.
Insurgents also said they seized Zliten from Kadhafi's forces, hours after saying they were in the town's centre, 150 kilometres (93 miles) east of Tripoli.
Rebels have been seeking to sever Tripoli's supply lines from Tunisia to the west and to Kadhafi's hometown of Sirte in the east, hoping to cut off the capital, prompt defections and spark an uprising inside Tripoli.
Meanwhile, a Tunisian defence official said Tunisian troops clashed with a group of armed Libyans overnight in the country's southwest.
An army patrol came under fire from men travelling in several 4X4 vehicles with Libyan registration plates in the Douz region, the official said.
No one was caught and the attackers were still being hunted Saturday by ground and air forces, the official said, adding there were no casualties on the Tunisian side.
With the rebels vowing to take Tripoli before the Muslim holy fasting month of Ramadan ends in late August, Italian Foreign Minister Franco Frattini urged the population of the capital to rise up against Gaddafi.
"We hope the people of Tripoli... understand the regime has harmed its own people and will therefore join a process of political change to cut off room for manoeuvre for Kadhafi's regime," Frattini said.
Meanwhile, the International Organisation for Migration said it was drawing up plans to evacuate thousands of migrants stranded in Tripoli because exit points have been cut off after a spate of rebel successes.
"There are already thousands of Egyptians who are ready for evacuation now, and what we are hearing is that every day there are more and more requests," IOM spokeswoman Jemini Pandya said.
For its part, the International Committee of the Red Cross reported a "rapid deterioration in the humanitarian situation" in several Libyan towns.
Meanwhile NATO, in its operational update for Friday, said it had hit targets in the vicinity of Tripoli, Zawiyah and Zliten, including nine military facilities around the capital.
Elsewhere, reports from Brasilia said clashes broke out late Friday at the Libyan embassy in the Brazilian capital between supporters of Kadhafi and his opponents.
And Human Rights Watch announced it had sent a "four-person team" to Tripoli and other sites in western Libya under government control, where "they engaged senior Libyan officials on human rights in the conflict and visited sites of NATO air strikes where civilians are alleged to have died."