Using Temporary Tables in postgres functions

When I try to use TEMPORARY TABLE within postgres functions (using 'sql'
as a function language), I can't because postgres can't find that
temporary table. Consider this example:

CREATE FUNCTION func1() RETURNS SETOF v_messages_full AS $BODY$
CREATE TEMPORARY TABLE tmpTbl
AS
SELECT
message_id
FROM
cached_messages
WHERE
billing_status = 2;

UPDATE cached_messages SET billing_status = 1 WHERE message_id IN
(SELECT message_id FROM tmpTbl);

SELECT
*
FROM
v_messages_full
WHERE
message_id IN (SELECT message_id FROM tmpTbl);
$BODY$ LANGUAGE 'sql' VOLATILE SECURITY DEFINER;

When I try to execute above CREATE FUNCTION statement postgres gives me
this:
ERROR: relation "tmptbl" does not exist
CONTEXT: SQL function "func1"

If I rewrite func1() using 'plpgsq' I have no troubles creating
temporary tables, I just need to use EXEC when referencing to those
temporary tables (which is cumbersome, but there it is).

Am I doing something wrong here, or there is no way of using temporary
tables within 'sql' written functions?

Mike