"We're looking more to irregular actions, less spectacular but which will allow us to keep going for a longer time," Pascal Galeote, secretary general of the Marseille port workers section of the CGT.
"There should be a full resumption of activity on Monday morning," he said. But he added that a new call for a 24 hour strike on Wednesday had been launched.
Cargo traffic partly resumed on Sunday after strike action which began on Thursday, but the key oil hub of Fos Lavera remained paralysed by the stoppage.
Around 20 ships were blocked at the quayside or elsewhere in the harbour on Sunday.
Workers have been protesting against government plans to privatise loading activities at state-run ports.
The plans are aimed at making labour practices more flexible and helping French ports recover the market share they have been losing steadily in recent years, but unions say they threaten job security for port workers.
The cabinet is due to examine the proposals, which would affect seven autonomous ports that are operated by state-owned companies, on Wednesday.
The strategic Fos Lavera port supplies crude to eight refineries in southeast France that have total capacity of around 800,000 barrels per day.
Crude supplies from the terminal also go to the 300,000 bpd Miro refinery in Germany and the 68,000 bdp Cressier refinery in Switzerland.