I am an Iranian citizen. I stayed in Canada for 8 months and got a student F1 visa to come to the US to attend graduate school beginning September 2008. At that time, when I was entering the US, the US officer at the US Port of Entry told me since I lived in Canada for a while, if I want to visit Canada for a duration less than a month I do not have to apply for the visa to come back to the US. Now, I am graduated but am still a post-doctoral research fellow at the same school and with my F1 visa. Recently I have been issued Canadian Permanent Residency and I need to land in Canada within the next 2 months.
Currently I have one year commitment to work for the University and I need to return to the US shortly after landing in Canada. My question is that whether I still need to apply for a visa to come back to the US or if I do not need a visa since I have previously lived in Canada?