The French government said it would see through planned pension reforms but said the new system that has sparked nationwide strikes would be introduced gradually and public concerns would be addressed.

Transport systems were paralysed for a fourth day on Sunday as unions at state railway SNCF and Paris public transport system RATP extended their strike against the changes.

"I am determined to take this pension reform to its completion and ... I will address people's concerns about it," Prime Minister Edouard Philippe told Le Journal du Dimanche.

"If we do not implement a thorough, serious and progressive reform today, someone else will do one tomorrow, but really brutally," he told the weekly publication.

Philippe has said he would present a detailed outline of the pension reform plan on Wednesday.

Deputy Environment Minister Emmanuel Wargon told radio France Info the government would be flexible about both the timelime and implementation of the reforms.

"Timelines may be relaxed if necessary and we may differentiate how each special pension system converges with the new system under different deadlines and terms," she said.

She said a date would be set to implement the new system but people's pension rights would be calculated proportionally based on how much time they had worked under the new and old systems.

"Some say that everybody will lose under the new system. Not everybody will lose. It will be rather positive for a significant part of French citizens," she said.

Philippe Martinez, the leader of the CGT union, said the CGT would fight until the government dropped the plan.

"We will continue until the plan is withdrawn," he told the JDD, saying the prime minister should "go back to square one."

France has one of the most generous pension systems among countries in the OECD grouping of industrialised nations.

President Emmanuel Macron was elected in 2017 on a platform to liberalise the economy and reform the pension system.

Macron wants to introduce a pension system with equal rights for everyone and to do away with a set of sub-systems under which some workers at SNCF, RATP and other institutions can retire in their early fifties, a decade ahead of others.

Unions plan a second demonstration on Tuesday, after a Thursday's first protest attracted 65,000 people in Paris and 806,000 nationwide, according to police figures.