Host key verification failed [sftp] while logging into AWS in batch [non-interactive] mode

I am trying to login to aws EC2 instance in batch mode through shell script, For that i ran following command

sftp -oIdentityFile="$private_key_loc" -b $(pwd)/new/ec2/batch.txt  ec2-user@"$var_public_ip"

which gives me:

Host key verification failed.
Couldn't read packet: Connection reset by peer

batch.txt

put /root/Desktop/hello.txt ./
chmod 664 hello.txt

i have earlier used the following command successfully to transfer the files to the same EC2 instance in interactive mode

sftp -oIdentityFile="$private_key_loc" ec2-user@"$var_public_ip"
put hello.txt ./
chmod 664 hello.txt