sftp with an encrypted key

To connect to an sftp site using an SSH private key, the standard advice is
to use

  set sftp:connect-program ssh -a -x -i /path/to/key

If the private key is unencrypted, this works fine.  (A key loaded in a
running ssh-agent also works fine, without the above.)  But if the key is
encrypted, the password prompt seems to be blocked:

$ lftp sftp://cygwin:@cygwin.com
lftp [email protected]:~> set sftp:connect-program ssh -a -x -i /path/to/key
lftp [email protected]:~> ls
`.' Delaying before reconnect: 30

The text

  `.' Enter passphrase for key '/path/to/key': XXXX

sometimes appears briefly on the screen.  Eventually the connection 
fails.

It seems that lftp is blocking the password input from the console.  Is 
this a known problem?  Any idea how hard it would be to fix?

lftp 4.6.0 in Cygwin x86_64.

Thanks,
Andrew

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