To give some more insight ....The application thread launches a "sh" process, which inturn launches a "sftp" process and
 "sftp" inturn launches a "ssh" process for communication.

The application thread calls popen() with the following command string.
sftp -oPasswordAuthentication=no -oStrictHostKeyChecking=no -oNumberOfPasswordPrompts=0 -oPort=22 -oIdentityFile=FILE.
ssh.prv userID@IP 

After it successfully launches the above commad, we see 3 processes in a normal working scenario.

388177965 proc/boot/sh          
388177967 usr/local/bin/sftp     
388182064 usr/local/bin/ssh

In the scenario, where we see the thread getting reply blocked on process "1"
we see the following processes.

388177965 proc/boot/sh          
388177967 usr/local/bin/sftp
388182064 (Zombie)

It is clear that SSH is getting killed and becomes a Zombie. But, we are not sure why SSH is getting killed abruptly 

Does anyone had a similar experience while using sftp/ssh application????

Any help is appreciated.