Visiting a hospital in Tripoli on Thursday, Al Jazeera's James Bays said he saw the bodies of 15 men suspected to have been killed a few days earlier as the rebels closed in on the Libyan capital.
"The smell here is overpowering," Bays said from the hospital where a number of bodies lay.
"I have counted the bodies of 15 men we were told there were 17 here. Two bodies were taken away by relatives for burial."
"We are told these men were political activists who have been arrested over the last few days and weeks and being held near the Gaddafi compound. When the opposition fighters started to enter the compound we are told they were killed.
"Everyone I have spoken to who has looked at these injuries, all the medical staff, they say they believe that the injuries they see on the bodies of these men have the hallmark of a mass execution."
Bays said there were no forensic scientists at the hospital. Doctors there had taken photos of the exit and entry wounds on the bodies, with the intention of showing it to an expert at a later stage.
Hunt for Gaddafi
The grisly discovery came amid rumours that rebels had surrounded a Tripoli building where Muammar Gaddafi, the Libyan leader now on the run, is reportedly hiding along with some of his sons.
"They are together. They are in a small hole," Muhammad Gomaa, one of the fighters involved in the battle near Bab al-Aziziya - Gaddafi's compound that was overrun by the rebels - told Reuters. "Today we finish. Today we will end that."
However, Al Jazeera's Sue Turton reporting from Tripoli, said "at the moment, these are rumours, we cannot confirm whether those reports are true or not".
Meanwhile, fierce fighting continued across the capital on Thursday, Al Jazeera's Zeina Khodr reported.
Fighting was concentrated along the perimeters of Bab al-Aziziya and in the neighbouring Abu Salim district, where Gaddafi reportedly released, armed and paid former prisoners to fight for his regime.
"Rebels have managed to enter the Abu Salim neighbourhood; clashes are taking place and rebels are pushing very hard," Khodr said.
Rebel reinforcements also streamed into Tripoli from other cities to join in the fight against remnants of Gaddafi's forces.
A rebel spokesman told Al Jazeera that "Libyan territory is 90 to 95 per cent under the control of the rebellion".
The rebels are determined to find Gaddafi, and have offered amnesty and a reward to anyone who kills or captures the 69-year-old Libyan leader.
In Benghazi, the National Transitional Council (NTC) told a news conference on Wednesday that Libyan business people had contributed $1.7m for the cash reward.
"The NTC supports the initiative of businessmen who are offering two million dinars for the capture of Muammar Gaddafi, dead or alive," Mustafa Abdel Jalil, the NTC chief, said.
Meanwhile, in an audio message broadcast on loyalist TV channels on Thursday, Gaddafi again called on his supporters to march on Tripoli and "purify" the capital of rebels, who he denounced as "rats, crusaders and unbelievers," Reuters reported.
In an earlier audio address broadcast on Wednesday by the al-Rai television channel, he said Tripoli residents should repel the rebels' advance.
Al Jazeera's Turton reported on Thursday that locals were very worried about attacks by pro-Gaddafi supporters across the city.
"There are check points popping up all over the city. Locals are managing to get hold of weapons to police their streets," she said.
"There is a lot of nervousness … people are very worried that Gaddafi loyalists are coming through these streets
"We've been told about clashes as rebels try to regain control of Abu Salim, the pro-Gaddafi neighbourhood that took a lot of casualties yesterday when rebels took on Gaddafi loyalists there."
The fight for Sirte
Elsewhere in the country, rebel commanders said they are readying fresh attempts to advance against Gaddafi's forces in his hometown Sirte, 360km east of the capital and to break a siege of Zuwarah, a town to the west.
Jacky Rowland, Al Jazeera's correspondent in Ras Lanuf, 200km from Sirte, said rebels there were assembling heavy weaponry in anticipation of an assault on the Gaddafi stronghold of Sirte.
However, Scott Heidler, Al Jazeera's correspondent in the eastern city of Benghazi, said there had already been a stop to rebel advancement towards the Gaddafi stronghold.
"So we are facing a battle in the coming hours," he said.
Rebels advancing towards Sirte were also blocked on Wednesday in the town of Bin Jawad as loyalists kept up stiff resistance.
Source WARNING: EXTREMELY GRAPHIC VIDEO AT SOURCE
Survivor tells of mass killing Also, obviously, upsetting descriptions.