Yes, you can track those registered mail numbers via USPS. Most of the time, until the item enters the US, the status will read "Origin post is preparing shipment." Once it enters the US, while it is in customs, it will read "Processed through sort facility" and the location where it entered. Then you won't get any updates for a while, until it goes through more sort facilities, and finally to your local post office and out for delivery. Sometimes, the system won't update from the initial waiting phase in customs, until you actually receive the package.
Format of the tracking number will be as: EM12 3456 789V N.