Different widths for different administrative boundaries with spplot() function

Hello friends,

I tried to create thematic maps using spplot() function for the NUTS2 
and NUTS3 administrative levels of the EU. In the maps I'm using, the 
lower level areas are subdivisions of the parent areas. Is it possible 
to make the country borderlines (i.e. NUTS1 level) thicker than the 
lower levels (regional) borderlines?

Thanks.

Tomas