The super Swede has been linked with Arsenal and Liverpool in recent days.
But Howe declared on Friday: "From my point of view he is not for sale.
"I want to build a squad, I want the strongest team possible for next season. There is a lot to be excited about here, hopefully. The end of the season is crucial for us. I think we have a strong squad and if we can add players for the first time in several windows, it will be a massive, massive difference for us.
"That's what I'm focusing on, not the opposite."
On Newcastle's plans for the summer market, Howe also said: "We want to keep our best players. I think I've made that very clear. It's not just my desire, it's everyone's desire at the club. I don't have a crystal ball, I can't sit here and say what's going to happen.
"I can only say what my desire is, and that is to build a team, the best team possible, and to do that you can't constantly lose your best players."