magorkhe,
I dont think this is a scam.IRS provides this website when u calll to their toll free number and I got that toll free number from the letter.
And the letter clearly mention my name ,address and social security number. if they are tryin to steal my identity and lets assume that letter is fake why they provides my SSN in that letter?
And regarding that website if u click exit it takes you to IRS website homepage.And this privacy policy says 

Privacy Notice

This service is controlled and operated by a third party on behalf of the IRS. Please visit the irs.gov site for the complete IRS Privacy Policy.

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Nejor,
I didnt have that kind of problem so the better option is you call them.