Islamist militants seized Al-Bahi and Al-Mabrouk fields on Tuesday and were heading for a third one, at Al-Dahra, said a spokesman for the Libyan oil industry's security service.
Violence and a slowdown at export terminals have already forced a shutdown at Al-Bahi and Al-Mabrouk, about 310 miles (500 kilometres) east of Tripoli, for the past several weeks.
An attack on the sites in February killed 11 people and all staff were evacuated.
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Libya has been awash with weapons since the 2011 uprising that toppled and killed Kadhafi, and opposing militias have since been battling for control of its cities and oil wealth.
It has two rival governments and parliaments -- those recognised by the international community sitting in the far east of the country and the others in the capital.
On Tuesday, security chief Colonel Ali al-Hassi said militia warplanes had attacked the major export terminal at Al-Sidra but were driven off without hitting their targets.
In response, planes of the internationally recognised authorities struck Tripoli's militia-controlled Mitiga airport without causing any casualties.