Explicitly Rendering ReCaptcha - Onload Function Not Firing

<html>
  <body>
    <p>ReCaptcha Test</p>

    <div id="recaptcha"></div>

    <script src="https://www.google.com/recaptcha/api.js?onload=recaptchaCallback&render=explicit&hl=iw" async defer></script>

    <script type="text/javascript">
      var recaptchaCallback = function () {
        console.log('recaptcha is ready'); // showing
        grecaptcha.render("recaptcha", {
            sitekey: 'SITE_KEY',
            callback: function () {
                console.log('recaptcha callback');
            }
        });
      }
    </script>

  </body>
</html>