Specify depth of vocabulary used in exposed filter

My goal is to display only root terms of a vocabulary inside an exposed filter.
My vocabulary is Region and each region has several sub-regions. In my exposed filter I want to display only regions without their sub-regions.
On Term Reference fields I have an attributes "depth" that lets me display only root parents. Views does not seem to provide this feature.
For now I have checked the "Limit list to selected items" option and selected manually each root terms. But this is not dynamic...
How are we supposed to do this ?
Thanks!