For anyone who has tried sudo puttygen ~/.ssh/your-key.pem -O private-openssh -o ~/.ssh/your-key-new.pem and got an error message saying puttygen: this command would perform no useful action there is an even newer format so you need to amend the command as follows:

sudo puttygen ~/.ssh/your-key.pem -O private-openssh-new -o ~/.ssh/your-key-new.pem

I was using a key generated by AWS on Manjaro which is a bit more bleeding edge than most other distros, still worked but the warning message was annoying.

For more info you can use man puttygen but the relevent section is below:

  -O output-type
    Specify the type of output you want puttygen to produce. Acceptable options are:

    private
      Save the private key in a format usable by PuTTY. This will either be the standard SSH-1 key format, or PuTTY's own SSH-2 key format.

    public Save  the  public key only. For SSH-1 keys, the standard public key format will be used (`1024 37 5698745...'). For SSH-2 keys, the public key will be output in the format specified by
      RFC 4716, which is a multi-line text file beginning with the line `---- BEGIN SSH2 PUBLIC KEY ----'.

    public-openssh
      Save the public key only, in a format usable by OpenSSH. For SSH-1 keys, this output format behaves identically to public. For SSH-2 keys, the public key will be output in the  OpenSSH
      format, which is a single line (`ssh-rsa AAAAB3NzaC1yc2...').

    fingerprint
      Print the fingerprint of the public key. All fingerprinting algorithms are believed compatible with OpenSSH.

    private-openssh
      Save an SSH-2 private key in OpenSSH's format, using the oldest format available to maximise backward compatibility. This option is not permitted for SSH-1 keys.

    private-openssh-new
      As private-openssh, except that it forces the use of OpenSSH's newer format even for RSA, DSA, and ECDSA keys.

    private-sshcom
      Save an SSH-2 private key in ssh.com's format. This option is not permitted for SSH-1 keys.

    If no output type is specified, the default is private.

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