There is “concerning” buildup of Russian forces near the border with Ukraine, according to a U.S. defense official.

Ukrainian Commander-In-Chief Ruslan Khomchack told parliament Tuesday that Russian troops from different regions have been assembling near the Russia-Ukraine border.

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

Ukraine and Russia have been in conflict since Russia annexed the Crimean Peninsula in 2014. The U.S. and 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.  

Relations between the two countries have been worsening. Just four days ago, four Ukrainian soldiers were killed by Russian separatist shelling in Eastern Ukraine. The U.S. Embassy in Kiev in a statement called on Russia to “observe ceasefire measures” and “end daily violence that is causing senseless suffering” and also urged Russia to end its aggression in Ukraine.

On Monday, National Security Adviser Jake Sullivan spoke with the head of the Ukrainian Presidential Office, Andriy Yermak, and affirmed the U.S.’ ongoing support for Ukraine’s sovereignty, territorial integrity, and Euro-Atlantic aspirations in the face of continuing aggression, according to a readout from the NSC.