How can I display a Browser Prompt for Confirmation before executing ajax callback for a button element?
This one should help you the same way you want to achieve the functionality.
Instead of ajax callback I have created a ctools modal which will return the delete and cancel button..

"delete/nojs/1" where 1 will be the Item which you want to delete.

use this in your form.

$form['delete'] = array(
  '#markup' =&gt; ctools_modal_text_button(t('Delete'),"delete/nojs/1", t('Delete'), 'ctools-modal-modal-popup-small'),
);


create a menu item.

function custom_module_menu() {
  $items = array();
  $items['delete/%ctools_js/%'] = array(
    'type' =&gt; MENU_CALLBACK,
    'title' =&gt; t('Delete'),
    'page callback' =&gt; 'delete_something',
    'page arguments' =&gt; array(1,2),
    'access callback' =&gt; TRUE,
  );
  return $items;
}


and call this function.

function delete_something($js = NULL) {
  if (!$js) {
    return drupal_get_form('delete_form');
  }
  ctools_include('modal');
  ctools_include('ajax');
  $form_state = array(
    'title' =&gt; t('Delete'),
    'ajax' =&gt; TRUE,
  );
  $output = ctools_modal_form_wrapper('delete_form', $form_state);
  if (empty($output)) {
    $output[] = ctools_ajax_command_reload();    
  }
  print ajax_render($output);
  drupal_exit();
}

function delete_form($form_state = NULL){
  $form['message'] = array(
    '#prefix' =&gt; '&lt;div class="delete_message"&gt;',
    '#value' =&gt; t('Are you sure want to delete?'),
    '#suffix' =&gt; '&lt;/div&gt;',
  );
  $form['submit']['delete'] = array(
    '#prefix' =&gt; "&lt;div id = 'delete_button'&gt;",
    '#suffix' =&gt; "&lt;/div&gt;",
    '#value' =&gt; t('Delete'),
    '#type' =&gt; 'submit',
    '#submit' =&gt; array('delete_form_submit'),
  );
  $form['submit']['cancel'] = array(
    '#prefix' =&gt; "&lt;div id = 'cancel_button'&gt;",
    '#suffix' =&gt; "&lt;/span&gt;&lt;/div&gt;",
    '#value' =&gt; t('Cancel'),
    '#type' =&gt; 'submit',
  );
  return $form;
}

function delete_form_submit($form, &amp;$form_state) {
 //some code to delete
}