"Regime militias have attacked Mislah oilfield and are heading towards Sarir oilfield farther south," Citizens for a Democratic Libya, a rebel-linked advocacy group, said in a statement.
"Nafourah oilfield, very near to Mislah, will be soon attacked too. All these supply oil to Tobruk," it said, urging both NATO and the United Nations to act to prevent the attack.
The allegations could not be independently verified.
On Friday the rebels` Transitional National Council said it had penned a deal with Qatar to market oil exported through the port of Tobruk, which is under rebel control, in exchange for food, medicine and -- it hopes -- weapons.
The barter deal is aimed at circumventing international sanctions imposed after the outbreak of the uprising against Kadhafi`s 41-year reign in late February, a revolt that has rapidly escalated into civil war.
Kadhafi`s forces retook the crescent of key oil terminals south of Benghazi last week for a second time and have since halted the rebels outside Brega.
Ali Tarhoni, a senior rebel council member in charge of oil and finance, said on Friday that the rebels could export up to 300,000 barrels a day through Tobruk, provided they secured vessels to transport it.
Qatar has yet to confirm the deal.
Libya`s oil exports, ordinarily about 1.7 million barrels a day, have ground to a halt since the unrest began.
While the exports meet only two percent of worldwide demand, Libya produces much prized "sweet," low-sulphur crude which is easy and cheap to refine into petrol. (*)