KYIV—Polish authorities accused Belarus of trying to spark a major confrontation on Monday and said they had mobilized additional soldiers as footage on social media showed hundreds of migrants walking toward the Polish border. In one video, shared by the Belarusian blogging service NEXTA, migrants carrying rucksacks and wearing winter clothing were seen walking on the side of a highway. Other videos showed large groups of migrants sitting by the road and being escorted by armed men dressed in khaki. “Belarus wants to cause a major incident, preferably with shots fired and casualties: according to media reports, they are preparing a major provocation near Kuznica Bialostocka, that there will be an attempt at a mass border crossing,” Deputy Foreign Minister Piotr Wawrzyk told Polish public radio. The European Union has accused Belarus of encouraging migrants from the Middle East and Africa to cross into EU countries via Belarus, as a …