Forces loyal to the Libyan leader, Muammar Gaddafi, have entered the oil port of Ras Lanuf in the east of the country and are fighting for control of the town, according to rebels.
Contacted by Reuters in the early hours from Brega to the east, Ibrahim al-Alwani said he and other rebel fighters in Ras Lanuf had seen government troops in the town centre.
"I saw maybe 150 men and three tanks," he said. "I can hear clashes."
Mohammed al-Mughrabi, who described himself as a spokesman for the rebels but declined to give his exact location, said government troops had landed by boat near the Fadeel hotel, where clashes were in progress.
"Four boats carrying 40 to 50 men each landed there. We are fighting them right now," he said.
Salam al-Burqy, another rebel fighter in Ras Lanuf, said the rebels had retreated but were still in control of the residential area of the town.
A fighter in Brega, who declined to be identified, said rebels in Ras Lanuf reported that government forces had entered by boat and in tanks.
On Thursday, government warplanes and gunboats bombarded rebel positions in Ras Lanuf, more than 300 miles (500km) east of Gaddafi's stronghold.
The bodies of four rebels killed in the fighting were brought to a hospital in Ajdabiya, further east, along with 36 wounded, said Ibrahim Saeed, a doctor at the hospital.
Rebels also reported an air strike on Brega, another oil port 50 miles to the east, on Thursday.
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