The advancing force, which is allied to a rival government based in Tripoli, moved east a week ago to try take the Es Sider and Ras Lanuf ports. The adjacent terminals have since closed, halting exports of an estimated 300,000 barrels a day of oil.
The recognized government of Prime Minister Abdullah al-Thinni was forced to relocate to the east after losing control of Tripoli in August to a group called Libya Dawn, which installed a new administration in the capital city.
On Sunday, pro-Thinni forces sent aircraft to bomb the advancing fighters some 40 km (25 miles) west of Es Sider and also inside Sirte, a large city further along the coast, said a military spokesman in Es Sider, informs Report citing Reuters.
He said the planes had bombed military targets, but Ismail al-Shukri, a spokesman for the rival force said civilian targets had been hit in Sirte.
There was no immediate word of any casualties.
The fighting is part of a wider struggle for control of the North African country which sits on Africa's largest oil reserves. Former rebel groups which helped topple Muammar Gaddafi in a 2011, NATO-backed uprising now fight each other.
Western powers fear the conflict could lead to the break-up of the OPEC producer. The United Nations had planned to launch a new round of talks to defuse the crisis last week, but the latest fighting delayed the negotiations. The U.N. said on Wednesday the venue and date for the meeting were still unclear.
A Reuters reporter visiting Es Sider saw Thinni's forces building up a defensive line with tanks and anti-aircraft gun mounted on trucks some 10 km (6 miles) west of the terminal.
"We are in contact with the airforce and the coastal guards to coordinate," said a commander. The reporter saw a helicopter and MiG jet standing ready at an airport at Ras Lanuf, which is normally used by oil firms.