France's premier will present new proposals Saturday on the government's hotly-contested pensions overhaul, seeking to appease unions waging a damaging, weeks-long transport strike.

After a day of meetings with labour bosses which he described as "very frank, very constructive and, I think very useful," Prime Minister Edouard Philippe said he would unveil "concrete proposals" Saturday "which could be the foundation of the compromise" being sought.

The country has been hit by 37 days of crippling train and metro stoppages as unions battle the proposals, one of President Emmanuel Macron's signature reforms.

The government is seeking to replace the existing 42 public pension schemes with a single, points-based system it says will be fairer and more transparent but which unions fear will see millions work longer for a smaller retirement payout.

After meeting Philippe, Laurent Berger of the moderate CFDT union said the prime minister had shown a "commitment to openness" regarding the hated proposal to make people work until 64 for a full pension -- two years beyond the current official retirement age.

"It's an unthinking measure which has no place being in the proposed law," Berger added.

But the CGT and FO unions, which demand the reform be dropped entirely, vowed to press ahead with the strike.

"Our determination remains absolute," said FO chief Yves Veyrier.

The government says the new "pivot age" of 64 would plug pension deficits set to soar in coming years as a growing number of retirees live ever longer.

Unions were told at Friday's meetings the measure would save five billion euros ($5.6 billion) by 2023 and some 11 billion euros by 2026.

The country has been hit by weeks of crippling train and metro stoppages as unions battle the pension proposals
The country has been hit by weeks of crippling train and metro stoppages as unions battle the pension proposals AFP / Martin BUREAU

Philippe has said he is willing to negotiate on the pivot age, but it remains a key feature of the draft overhaul bill.

He said Friday "good progress was made" in the latest round of talks held on the eve of a fifth mass demonstration called since the labour action began on December 5.

The prime minister as to meet Macron and other senior members of the ruling party Friday evening "to evoke possible advances", and present the new proposals to unions in writing on Saturday.

Saying he was hopeful of a compromise, Philippe nevertheless reiterated the government's commitment to the pension revamp and to present the plan to the cabinet by January and the National Assembly by February 17.

Some 452,000 people turned out across France on Thursday for marches that were marred by clashes with police and vandalism, with 27 people detained in Paris.

On top of Saturday's planned rallies, more protests have been called for three days starting next Tuesday.

"We have to step up our actions, and other sectors have to take up the baton," Benoit Teste of the FSU teachers' union said late Thursday.

The hardline CGT union, the largest among public sector workers, said it has extended until January 16 a blockade of refineries and fuel depots, which has already caused fuel shortages in some areas.

Commuting has become a daily headache as train services and Paris metro lines were again curtailed Friday.

On Saturday rail operator SNCF says about 80 percent of TGV trains will be running, and some 40 percent of suburban Transilien lines. But RATP in Paris says all metro lines will be open during the day.