I may not be totally right but you can apply directly through your sponsor for the extension. "The applicant's sponsor organization is responsible for submitting the new DS-2019 before the original expires. The best plan is to request an extension of the visa on the day that you learn that you are going to need to be in the country for a longer period of time. It doesn't matter if your J1 visa isn't going to expire until after several months have passed, it is much better to be on top of it."
Thank you awais,so can I apply for extension in USA without traveling back to pakistan?
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