German prosecutors have charged five employees of a British recruitment agency working with Ryanair over millions of euro of allegedly unpaid social security payments for its pilots.

No Ryanair staff were among the accused but "investigations against employees of Ryanair over incitement to the alleged activities continue," the prosecution service in the western city of Koblenz said.

The five employees of two British-based recruitment agencies allegedly withheld a total of about €6m in social security contributions for 277 pilots flying German routes for Ryanair.

News of the indictments came as the no-frills carrier faces the threat of more labour unrest in Germany and other European countries where pilots have vowed to fight for better pay and conditions.

No trial date has been set yet for the five unnamed accused, who are aged 35 to 62 and who could face up to five years in jail or fines if found guilty, the prosecutors said in a statement.

The accused allegedly withheld social security payments in 920 cases between January 2007 and June 2016.

The charges were filed in September but only now publicised after the accused had been informed, said the prosecutors.

Ryanair said in a statement that it was "fully assisting the Koblenz Prosecutor's Office with its investigations into a small number of contractor pilots who may have calculated their social insurance/tax incorrectly".

The airline was hit by a limited "warning strike" last Friday called by German pilots' union Cockpit, but passengers faced few disruptions and no cancellations.

Ryanair, Europe's second-largest airline by passenger numbers, later praised crew for "largely ignoring" the unprecedented walkout.

The airline had days earlier taken the unprecedented step of offering to finally recognise unions after crew in Ireland, Germany, Britain, Italy, Spain and Portugal threatened walkouts in long-running rows over pay and conditions.