Hi
The problem has been fixed now 
It was because of proxy server used in my organisation.
Adding below settings to web config did the trick.
<system.net>
<defaultProxy useDefaultCredentials="true">
<proxy proxyaddress="yourproxyaddress" bypassonlocal="True"/>
</defaultProxy>
</system.net>
Hope this helps some one who might encounter this error in future.
Thanks,
Vinayak