[XeTeX] Diacritical accent marks in malayalam, Tamil

[P. P. Narayanaswami]

I am unable to insert diacricical marks for characters in Mmalayalam.
For examples, 

\def\mal{\fontspec[Script=Malayalam]{Rachana}}

\.{\mal സ} does not propduce a dot over  സ, 
\d{ ഗ}  does not place a dot under  ഗ 
The same problems with Tamil also.
How can I place proper diacritical marks using the accent mark control sequences
\.{X}, \d{X}, and so on in Malayal;am, tamil, Telugu?