A spokesman for the Petroleum Facilities Guard, the militia that controlled the ports for months, denied the claim but acknowledged that they had come under attack Sunday morning. He is allied with the parliament based in eastern Libya, which refuses to recognize a newly-formed, UN-backed government. The resumption of oil exports would help address Libya's severe cash crunch.
Masmari said clashes were continuing at another oil port, Zueitina, and around the nearby town of Ajdabiya.
The HoR's forces are led by commander-in-chief General Khalifa Haftar, a former renegade army man under Muammar Gaddafi who is now thought to have backing from the UAE and Egypt.
Since the PFG took control of the ports they have been largely out of action in a dispute over pay.
A unity deal brokered by the United Nations, signed by rival factions in December despite opposition from hardliners, was meant to end the divide.
The GNA late Sunday called on all forces loyal to it to "protect and defend" the ports against what it called "flagrant aggression" against Libyan sovereignty.
It warned overnight that the country was at a "critical juncture", adding "the hopes of Libyans for stability in the country have been dashed".
While the West views the GNA as the best prospect at uniting Libya after five years of violence and political chaos, members of the Tobruk-based parliament have expressed serious reservations about the make-up of the GNA cabinet and have called for greater representation from eastern-based lawmakers.
Sunday's assaults triggered fears about a wider battle over Libya's lucrative oil resources - the nation has the world's ninth-largest oil reserves - undermining its ability to restart production and bring in revenue vital to reconciling rival militias and reconstruction.
The seizure of the ports deals a heavy blow to the unity government, depriving it of a key source of income.
They had been closed following attacks in January by the jihadist Islamic State group, who took advantage of turmoil after the 2011 revolt to gain a foothold in the country.
Ahmad Mesmari, a spokesperson for Haftar's forces, told a press conference that another oil terminal at Zuwaytina was not yet under their control.
In recent weeks, pro-GNA forces have been pressing to expel the last IS jihadists from what was their North African stronghold of Sirte. That western city's factions have provided crucial support to the GNA, and many remain fiercely opposed to Haftar.