VATICAN CITY (Reuters) -Pope Francis on Monday said future generations must not be overloaded with the cost of today's pension systems, in apparent support for reform at a time when the issue is politically explosive in France.

The 86-year-old leader of the Roman Catholic Church spoke in general terms, without referring to France, in a speech to leaders and staff of Italy's welfare agency INPS at the Vatican.

"To think that in some countries our grandchildren will be born with a terrible public debt. The choice of sustainability, on the other hand, responds to the principle that it is unfair to put irreversible and too heavy burdens on young people," he said.

France has been rocked by strikes and sometimes violent protests against a reform that President Emmanuel Macron has pushed through, bypassing parliament, which raises the retirement age by two years to 64.

Other European countries, facing the challenge of longer life-expectancy and falling birth rates, have adopted similar cost-saving measures, with governments defending them on financial sustainability grounds.

Francis, who last year said that choosing pets over kids is selfish, said an unnamed Italian man in his 60s had once told him: "Who will pay for my pension? It won't be the little dogs that people have instead of children."

In his speech, he called for "wise politicians" who can distinguish between good and bad times and who "avoid wasting resources ... and do not leave future generations in dire straits".

The pope also stressed the contribution made to pension systems by foreign workers, and said people should have access to dignified jobs, without excessive forms of temporary contracts or being forced to accept undeclared work.

(Reporting by Alvise Armellini, editing by Crispian Balmer and Nick Macfie)