savepoint and rollback are not working in postgresql script

I am using savepoint and rollback in a my below pg script. Getting below error.

ERROR: unsupported transaction command in PL/pgSQL

Please help me how to achieve this.

do
$block$ 
declare
BEGIN
 perform updatecustdtls (
                  custdes               => 'initial state',         
                  custno            => 'C1122',
                  cmd       =>  TO_TIMESTAMP('11/21/2005','dd/mm/yyyy')::TIMESTAMP(0),
                  );   


 savepoint updatecust;                         

update custd set custid = 9 where custno = 'C1122';


    perform updatecustdtls (
                  custdes               => 'middle state',         
                  custno            => 'C4455',
                  cmd       =>  TO_TIMESTAMP('11/11/2006','dd/mm/yyyy')::TIMESTAMP(0),
                  );

rollback to updatecust;  

    perform updatecustdtls (
                  custdes               => 'final state',         
                  custno            => 'C88809',
                  cmd       =>  TO_TIMESTAMP('08/15/2007','dd/mm/yyyy')::TIMESTAMP(0),
                  );

end $block$; 

Thanks in advance.