Error ora-01653

Hi. I keep getting this error from oracle, and am rapidly running out of ideas
for fixing it. Does anyone know what may be causing it (lack of space is not)?

Thanks in advance,

Richard

sqlplus output:

SQL*Plus: Release 3.3.2.0.0 - Production on Tue Feb 9 13:58:47 1999  

Copyright (c) Oracle Corporation 1979, 1994. All rights reserved.    

Connected to:
Oracle7 Server Release 7.3.2.1.0 - Production Release With the parallel query and Spatial Data options PL/SQL Release 2.3.2.0.0 - Production  

SQL> insert into keyword(keyword,url_uid) values ('fred',-1); insert into keyword(keyword,url_uid) values ('fred',-1)

            *
ERROR at line 1:
ORA-01653: unable to extend table HPAROB.KEYWORD by 3596 in tablespace USERS Received on Tue Feb 09 1999 - 08:01:29 CST