Hartlepool have signed Dutch winger Sidney Schmeltz on a short-term deal.
Charlton Athletic sign USA defender Oguchi Onyewu on a short-term contract until January.
This season Tello has made 16 appearances in all competitions for Barca, having represented Spain at this summer's Olympics.
A Barcelona statement read: "Barcelona announce they have reached an agreement with the player Cristian Tello to extend his contract until June 30, 2016. His buy-out clause has been set at 10 million euros.
"The player will sign his contract on Monday, December 17, in the Barcelona offices."