French trade unions were left unconvinced Wednesday when Prime Minister Edouard Philippe set out the details of the government's pension reforms plan.

Amid days of strikes that have crippled public transport, Philippe insisted that the planned single national system would be based on solidarity and would be fairer than the multiple existing schemes.

"France has not chosen, and I think it never will choose, the way of 'each for himself and the Devil take the hindmost,'" Philippe told the country's Economic, Social and Environmental Council.

The new system would only apply to those born in 1975 or later, he said, a concession compared to initial proposals that those born from 1963 on could be affected.

Transport workers have been at the heart of strikes against the reforms, with the Paris metro mostly closed and trains severely disrupted Wednesday for the seventh day running.

The special retirement regimes would be phased out "gradually, without brutality, and with respect for individual careers," Philippe said.

UNSA union leader Laurent Escure said later that he was "still waiting for a lot of clarification" on what the reforms would mean in the sector.

Philippe also insisted that while the legal retirement age would remain at 62, there would in future be discounts to the pensions of those retiring before age 64, and bonuses for those staying later. "We will have to encourage the French to work longer" to ensure the system was sustainable, he said.

But that got an angry response from the head of France's biggest trade union, the moderate CFDT, which has so far mostly backed dialogue with the government rather than strike action.

Philippe had "crossed a red line" by including the issue of retirement age in the reform package, CFDT secretary general Laurent Berger said. "I think this reform has been weighed down with an excessive budgetary aspect, and we are going to try and get rid of that ballast in the coming weeks," he added.

Philippe Martinez, head of the hardline CGT union which has been a driving force in the protests, meanwhile told LCI television the announcements showed the government "couldn't care less about anyone."