Postgres trigger issue with update statement in it.

 Hello Friends,
I am new to Postgres DB. Recently installed Postgres 9.2. 
Facing an issue with very simple trigger, tried to resolve myself by reading documents or google search but no luck.

I have a table A(parent) and table B (child). There is a BEFORE INSERT OR UPDATE trigger attached in table B. This trigger has a update statement in it. This update statement should update a respective record in table A when ever there is any insert/update happen in table B.  The issue here is where ever I insert/update record in table B, getting an error as below :

********** Error **********
ERROR: cannot begin/end transactions in PL/pgSQL
SQL state: 0A000
Hint: Use a BEGIN block with an EXCEPTION clause instead.
Context: PL/pgSQL function func_update_payment() line 53 at SQL statement

Line no 53 in the above error message is an update statement. If I comment out the update statement, trigger works fine.

Can anyone shed some lights on this? Your help is appreciated.

Thanks and Regards
Kaleeswaran Velu