SQL & PL/SQL

NLSRTL 11.2.0.3.0 Production
Oracle Database 11g Enterprise Edition 11.2.0.3.0 64bit Production
PL/SQL 11.2.0.3.0 Production
TNS for IBM/AIX RISC System/6000: 11.2.0.3.0 Production

I have a database user U1 and the user has been granted the role DBA

connect u1/<password>@db

SELECT * FROM user_role_privs;

The output is:

U1	CONNECT                             NO  YES  NO
U1    DBA                                     NO  YES  NO
U1    EXECUTE_CATALOG_ROLE      NO  YES  NO
U1    EXEC_SYS_PACKAGES_ROLE   NO  YES  NO

If I execute with the same user U1 call to the package DBMS_MONITOR it works fine:

begin
  DBMS_MONITOR.SESSION_TRACE_ENABLE(waits => TRUE, binds => TRUE);
end;
/

If I try to make a call to the package DBMS_MONITOR within the body of a function/procedure/package owned by the same user U1, it fails to compile with the error:

create or replace procedure trace_proc
is
begin
  DBMS_MONITOR.SESSION_TRACE_ENABLE(waits => FALSE, binds => TRUE);
end trace_proc;
/

The compilation fails with error: [Error] PLS-00201 (4: 3): PLS-00201: identifier 'DBMS_MONITOR' must be declared

I tried the same by prefixing the package with "SYS" but it still fails to compile.

Can you please give some clues why is it happening?

Thank you