SQL & PL/SQL

Hi Every One.
I am trying to use an UPDATE query as dynamic SQL with the 'FOR UPDATE' and 'WHERE CURRENT OF' clause. Let me explain you 2 scenarios where its works fine and 1 scenario it doesnt.

NOTE: I AM USING 'FOR UPDATE' KEYWORD IN THE CURSOR DECLARATION, BECAUSE OF WHICH SCENARIO 2 WORKS FINE.

Scenario1 **WORKS**:

for v_cur in cur loop
v_update_query :=  'update temp set '
         || v_col_name
         || ' = '
         || v_col_value;
execute immediate v_update_query;
end loop;

Scenario2 **WORKS**:

for v_cur in cur loop
update temp set  col1 = 10 where current of cur;
execute immediate v_update_query;
end loop;

Scenario3 **DOES NOT WORKS ** Error Message "ORA-03001: unimplemented feature":

for v_cur in cur loop
v_update_query :=  'update temp set '
         || v_col_name
         || ' = '
         || v_col_value      
         || ' where current of cur';
execute immediate v_update_query;
end loop;

I also tried putting it like this but does not work: ' where current of ' || cur;
The Error Message is "PLS-00306: wrong number or types of arguments in call to '||' "

Where am I doing wrong. Kindly let me know.
Thanks in advance.
Aj