Roman, thank you for your comments. I have gone through a couple of relevant articles found on the net. As the bird in question was probably a 1st year juvenile, it will be even more difficult to ID; a juv. ocularis looks very alike a juv. lugens. For the time being, I am inclined to think this was a White Wagtail ocularis as the greater coverts were darker than those of a lugens. Nevertheless, more expert opinions will be much appreciated as I am just an ordinary birder.