Unverified footage posted on the internet showed huge blazes and thick black smoke billowing in the sky in Zawiya city.
The refinery is still functioning without any suspension, two engineers working at the refinery said.
The incident came two days after a drone crashed into an untreated naphtha tank at the Zawiya refinery early on Saturday, causing a leak that staff managed to control.
A fire, accompanied by heavy smoke, broke out on Monday in a diesel tank at Libya's Zawiya refinery after the tank was struck under circumstances whose cause remains unknown, Brega Petroleum Marketing Company said in a statement.
Brega is owned by state oil firm NOC and is in charge of fuel supplies.
Zawiya refinery, the largest in the country as Ras Lanuf is out of operation, around 40 km (25 miles) west of Tripoli, has a capacity of 120,000 barrels per day. It is connected to the country's 300,000 bpd Sharara oilfield.
Firefighter brigades are working on containing the blaze, the company said, adding that it was assessing the damage and would provide updates once confirmed information became available.
The tank, with estimated 4.5 million litres of gasoline, "was directly targeted," NOC said in a statement, declaring the extreme state of emergency in the area.
It demanded competent authorities to "immediately intervene" and launch investigations into the incident.
