Dependent drop-down list from map in Thymeleaf

I want to create a drop-down list with countries and the second drop-down list with cities, which depends on selected value in the first list. And the list of cities should be changed dynamically. In the view (Thymeleaf) I have a Map<CountryModel, Set<RegionModel>> from controller. CountryModel's name should be shows in the second drop-down list, and Set should be shows in the second(dependent) drop-down list.
Here I create first drop-down list:

 <tr>
   <td th:text="#{country}"/>
   <td>
      <div class="form-group">
          <select th:field="*{transferRequestModel.country}" class="form-control" id="country">
                <option th:each="country : ${transferModel.countries}"
                    th:value="${country}"
                    th:text="${country.key.countryName}">Wireframe
                </option>
          </select>
      </div>
    </td>
 </tr>

So how to create second drop-down list which depends on selected country in the first list?