Forces loyal to Libya's internationally recognised government carried out their first air strikes on Sunday against the country's militia-held third city of Misrata, a spokesman said.

Colonel Ahmed Mesmari said the raids were a response to a renewed attempt early Sunday by the Fajr Libya (Libya Dawn) militia to seize the key Al-Sidra oil export terminal.

Residents said the strikes targeted a flying school close to Misrata airport, the port and a steel plant.

There were no immediate reports of casualties.

Mesmari said the strikes took place after Fajr Libya launched an air raid on Al-Sidra using a Mig-23 jet that took off from the aviation school in Misrata.

"That is why we targeted it and other positions of the militia," he said.

The UN mission in Libya, UNSMIL, issued a statement condemning the air strikes, warning that failure to take steps "towards de-escalation" will lead to "all-out war" in Libya.

"This cycle of violence... if continued, will lead the country to chaos and all-out war," it said, urging the warring sides to take "courageous steps" to end the fighting.

Fajr Libya has been trying to take Al-Sidra and the nearby Ras Lanuf terminal since Thursday when it killed at least 22 soldiers in a surprise attack by speedboat.

Seven oil storage tanks at Al-Sidra were set on fire as a result of the fighting.

On Sunday firefighters managed to extinguish four of the fires, an oil official said, adding that blazes still raged in three tanks.

Since clashes first erupted around the export terminals on December 13, Libya's oil production has dropped to less than 350,000 barrels per day compared with 800,000 previously, industry experts say.

More than three years after dictator Moamer Kadhafi was toppled and killed in a NATO-backed revolt, Libya is awash with weapons and powerful militias, and has rival parliaments and governments.

As well as Misrata, the capital Tripoli and second city Benghazi are largely in the hands of militias, and the internationally recognised government has taken refuge in the remote east.

- Trial adjourned -

Elsewhere, a Tripoli court on Sunday delayed yet again the trial of some 40 members of Kadhafi's ousted regime, including his son Seif al-Islam, a judicial source said.

The accused, who also include Kadhafi's former spy chief Abdullah Senussi, have been charged over their roles in suppressing the 2011 uprising that eventually toppled Kadhafi.

All of the defendants are accused of murder, kidnapping, complicity in incitement to rape, plunder, sabotage, embezzling public funds and acts harmful to national unity.

Only 31 of the accused appeared in court on Sunday, including Baghdadi al-Mahmudi, Kadhafi's last premier, an aide to the prosecutor said.

Seif al-Islam was not present as he has been held since his capture by rebels in November 2011 in the hilltop town of Zintan, west of Tripoli.

Seif, 42, is wanted by the International Criminal Court (ICC) on charges of crimes against humanity relating to the bloody repression of the 2011 uprising.

On December 10, the ICC referred Libya to the UN Security Council for not handing him over for trial.

A new hearing in the case of the 40 is due to take place on January 11, when defence lawyers are due to speak.