How to display alert message from javascript inside the ASPXCallbackPanel

protected void OnCallback(object source, DevExpress.Web.ASPxClasses.CallbackEventArgsBase e) 

{        

         ASPxCallbackPanel callbackPanel = (ASPxCallbackPanel)source;       

          callbackPanel.Controls.Add(new LiteralControl("My New Content"));        

         WebControl script = new WebControl(HtmlTextWriterTag.Script);        

      callbackPanel.Controls.Add(script);        

      script.Attributes["id"] = "dxss_123456";        

    script.Attributes["type"] = "text/javascript";        

     script.Controls.Add(new LiteralControl("var str = 'test'; alert(str);"));    

}