"Both players will be at [head coach] Julen Lopetegui's disposal... RFEF [The Royal Spanish Football Federation] has notified the clubs that both footballers will join the team in their trip to Russia, without a specific return date,” the team said in a statement published on its official website.

Hernandez and Vallejo will train with the squad before the team plays Tunisia in a friendly on Saturday.

The game is set to take place in the southern Russian city of Krasnodar, where the squad will be based during the World Cup.

21-year-old Vallejo is currently playing for Real Madrid, while Hernandez, also aged 21, is set to join Atletico Madrid as the club has recently reached an agreement with Villarreal for his transfer.

Russia will host its first World Cup from June 14 to July 15. The fixtures will be played at 12 stadiums across 11 cities. 

2010 World Cup winners Spain were drawn in Group B where they will play Portugal in Sochi on June 15, Iran in Kazan on June 20 and Morocco in Kaliningrad on June 25.