A rocket set a storage tank at the eastern Libyan oil port Es Sider on fire as armed factions allied to competing governments fought for control of the country's biggest export terminal, officials from both sides said on Thursday.

Clashes were also reported from Sirte, a city west of Es Sider, killing up to 19, among them 14 guards at a power plant, residents said. No more details were available.

Es Sider and its adjacent Ras Lanuf terminal have been closed since a force allied to a rival government in Tripoli moved east trying to take them, part of a struggle between former rebels who helped topple Muammar Gaddafi in 2011 but now fight for power and a share of oil reserves.

Libya has had two governments and parliaments since a group called Libya Dawn seized the capital in August by expelling a rival faction, installing its own prime minister and forcing the recognized premier, Abdullah al-Thinni, to operate out of the east with the elected House of Representatives.

Thinni accuses Libya Dawn of relying on Islamists. The Tripoli-based government says Thinni's forces have allied themselves with former Gaddafi officers such as ex-general Khalifa Haftar.

Ali Hassi, spokesman for a security service allied to Thinni's internationally-recognized government, said the rival force tried taking the Es Sider with speedboats, opening fire on an oil tank.

Ismail al-Shukri, spokesman for the rival force, said it had launched a new push to take the ports but denied it had sent boats or shot at any storage tanks. He blamed the other side, saying aircraft had attacked the tank.

"A tank was hit but the damage is limited," said Hassi, adding that the advance had been stopped with aircraft and ground troops. Clashes were also reported in Ben Jawad west of Es Sider.

An oil ministry official said the storage tank was still on fire. Smoke could be seen on pictures posted on social media websites and described as showing the port.

The fighting has reduced Libya's crude output to 352,000 barrels a day, a spokesman for state National Oil Corp said. Only the Brega, Sarir, Messla and offshore operations were still producing, he said. Es Sider and Ras Lanuf ports had been processing an estimated 300,000 barrels per day.