syntax error with execute

I have a query like this in a plpgsql function:

EXECUTE 'INSERT INTO '||tablename||' ('||fields||') VALUES
('||vals||') RETURNING currval('''||seqname||''') INTO newid'

and I get the response:

ERROR:  syntax error at or near "INTO"
LINE 1: ...','2008','4',NULL) RETURNING currval('id_seq') INTO newid

And I do not understand this error. If I take the INSERT command and
run it by hand, it works fine, but it doesn't work in the function
when called by execute. Anybody has an idea on what is wrong and what
to do about it?