However, the unions said on Thursday workers of three other plants had refrained from joining the strikers immediately, easing fears of an immediate shutdown of the region's oil processing capacity.
Talks between Fos Lavera strikers and the management of the Mediterranean port were still in a deadlock and no meetings were planned, the port said as the strike entered its 11th day and dozens of vessels with oil and oil products remained blocked.
Workers are protesting against a port reform as well as French President Nocolas Sarkozy's pension reform, which will increase retirement age to 62 from 60, and has already sparked several waves of nation-wide protests.
Benchmark European gasoline prices eased a notch to hover near a five-month high at $762 a tonne on Thursday with oil trading slightly higher at $83.74 CLc1.
France could be grounded to a halt on Oct. 12 when railway, refinery, power, and underground workers will go on strike.
The port serves eight plants which can process more than 1 million barrels per day -- around 7 percent of Europe's total capacity -- although a refinery in Germany can be supplied by alternative routes.