There is "concerning" buildup of Russian forces near the border with Ukraine, a U.S. defense official told CBS News after Ukraine's Commander-In-Chief Ruslan Khomchack told his country's parliament that Russian troops from different regions have been assembling near the border.

One possible explanation is that the troops are training, but the American defense official said Russia had not announced any exercises in the area.

Ukraine and Russia have been in conflict since Russia annexed the Crimean Peninsula in 2014. The U.S. and its NATO allies do not recognize Russia's annexation of the peninsula, and the U.S contributes millions in both non-military and military aid each year to Ukraine. 

Read more: CBS News