SQL & PL/SQL

Hi Experts,

In while loop; i am trying to use continue / or CONTINUE when

eg:
CONTINUE when N_BILL_GROUP_ID IS NULL;
or
IF N_BILL_GROUP_ID IS NULL THEN
CONTINUE;
END IF;

But i am getting error "Encountered the symbol "WHEN " when expecting one of following
or identifier continue must be declared.

Why i am unable to use continue.

Thanks,
VS