counter spell of the Balrog

[tinewelt]

at the bridge of Khazad-dum, Gandalf speaks of his first initial struggle with the balrog, although he does not know what it is yet. he says

What it was I cannot guess, but I have never felt such a challenge. The counter-spell was terrible, it nearly broke me.

Is this explained anywhere else in tolkiens writings? Do Balrogs have the power of spells and some magic? Or is it just a metaphor referring to the evil of the Balrog? He also said

Something came into the chamber- i felt it through the door. it laid hold of the iron ring, and the it perceived me and my spell

I am just wondering if this is an only instance or is it explained elsewhere about these powers