PARIS (Reuters) - France suffered a third day of transport disruptions on Friday as rail workers kept up a strike campaign against pension reform and the government and unions skirmished over how to resume negotiations.

The open-ended strike has developed into a trial of strength over one of the key economic reforms promised by President Nicolas Sarkozy and grass-roots union resistance appears to have hardened as workers called for the stoppage to continue.

Trains are expected to remain blocked until at least the weekend after unions said they were still waiting for an agreement on the conditions under which three-party talks between government, unions and employers could take place.

Labour Minister Xavier Bertrand has insisted that workers must go back to work before talks can take place, but he said on Friday that they could begin as long as union leaders called for an end to the strike.

"There needs to be a call to return to work from the organisations in the companies for us to have an immediate -- you understand me -- an immediate opening of the three-party negotiations that the unions have asked for," he told RTL radio.

There was a slight increase in the number of trains and Paris metros running, but services were still heavily disrupted and commuters faced an uncomfortable journey into work.

About one in three high speed TGV trains from Paris were expected to be running, but TGV services between provincial centres were halted and there were reduced regional services.

The Paris transport authority RATP said services were still heavily affected but most metro lines were running at least a reduced service and a third of buses and trams were running.

Unions oppose plans to scrap special pension privileges that allow some 500,000 public sector workers to retire on full pensions after paying contributions for only 37.5 years, instead of 40 years for other workers.

The government says the so-called "special pension regimes" are outdated, unfair and unaffordable. Unions say the benefits make up for often awkward and difficult working conditions.

Opinion polls show that most French people support reform of the system, but with separate protests by students and civil servants brewing and widespread concerns over the cost of living, the protests could widen if the strikes drag on.

France's SNCF railway company said just over 40 percent of its staff were on strike on Thursday, against more than 60 percent on Wednesday, but a hard core of resistance in local union branches remained determined to keep up the campaign.

"We're at a moment where we need details and explanations on the content of negotiations, on the method and timetable," Didier Le Reste, head of the rail division of the CGT union told France Info radio.

"We're still waiting for a response from the companies and the government on this," he said.

(Editing by Crispian Balmer)