Accessing sub diagonals / spdiag in R ?

Hello, I'm attempting to access a specific number of sub diagonals in a
MATRIX and have been accustomed to using spdiags in MATLAB or Octave. I've
got a solution pieced together using for loops and it works though isn't
vectorized and liable to run very slow
for large matrices.

As an example:

A =
1 2 3 4 5
9 8 7 6 5
4 5 6 7 8
5 4 3 2 1
8 7 6 0 1

The subdiagonals are: 9,5,3,0 ? 4,4,6 ? 5,7 ?and 8,
I know about lower.tri and can fetch the data in a resulting vector
which ,in this case, would be:

9,4,5,8,5,4,7,3,6,0

though I would have to manipulate this some more to extract the other
diagonals (imagine this being done for say a 1000 x 1000 matrix). I looked
at CRAN and didn't see anything corresponding to
spdiags. The closest package appeared to be the one relating to sparse
matrices and band symmetry. Would you have any suggestions about 1) how to
emulate spdiags or 2) working with the lower.tri returned-data and
extracting the remaining diags efficiently. I can live with what I have but
imagine that there is something more direct. Thanks