Icelandic/Old Norse -kk- suffix

Hello,

Does anyone happen to know the origin of the suffix -kk- in Icelandic/Old Norse (as seen in hækka "rise, ascend" < hár "high", fækka "lessen" < fár "few" etc.)?

I suspect that it's from the widespread *-sk- verb suffix, but I wanted to confirm whether this is the case.

Thanks