begin update ... syntax error

I was trying to speed up a simple update query

fri=# begin
fri-# update tabA set nobegin=tmp.nobegin, noend=tmp.noend
fri-# from tmp
fri-# where tabA.acc=tmp.acc;
ERROR: syntax error at or near "update" at character 7

The same query can be run with no problem if not starting with BEGIN.

Does mean that BEGIN cannot preceed UPDATE?

Or I am making some obvious miskate?

Kemin

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