Train drivers with the Spanish national rail operator Renfe Wednesday announced four one-day strikes in the first two weeks of September citing a lack of progress in their collective bargaining over pay and conditions.

The train drivers' union Semaf called strikes on Sept. 4, 11, 14 and 15, adding they "did not rule out" other unions supporting their action.

"It has left us no alternative other than to call a strike," the union said.

Semaf's decision comes in the wake of a statement by the union Comisiones Obreras which estimated that Renfe needed at least 1,125 more workers to deal with what it describes as "the lack of maintenance" on engines and rolling stock.

The strike announcement comes just days after figures showing Renfe had 700,000 extra passengers on its high-speed trains during the first six months of 2015 and that the number of rail travelers had outstripped those preferring to fly by around 410,000 passengers.