Frost covered mountains of Phja Oắc in the northern province of Cao Bằng, Đồng Văn in Hà Giang Province, Mẫu Sơn in Lạng Sơn Province, and Y Tý in Lào Cai Province on Sunday as temperatures dropped below zero degrees Celsius. A strong cold front plunged the northern region to the lowest temperatures this winter. VNA Photos