Ukraine is under attack from Belarus as well as Russia, border guards tell AFP.
Troops are entering Ukraine from a border crossing with Belarus, CNN reports.
The CNN report is based on footage from a livestream of a camera in the area.
The outlet says troops are entering at the Senkivka, Ukraine crossing with Veselovka, Belarus.
Tens of thousands of Russian troops are in Belarus.
The military exercises by Russia in Belarus were supposed to end last weekend but Minsk then announced that the troops would remain to carry out more maneuvers for an unspecified duration.
The Ukrainian capital of Kyiv lies just 150 kilometers (90 miles) south of the Belarusian border, while the northern Ukrainian city of Chernigiv is a mere 60 kilometers (40 miles) east of Belarus.