▶ Watch Video: How Bellingcat tracked a Russian missile system in Ukraine 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. An Ukrainian serviceman stands holds a position on the front line with Russia-backed separatists in Ukraine’s eastern Donetsk region, February 19, 2021. ANATOLII STEPANOV/AFP/Getty 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. Relations between the two countries have been getting worse. Just four days ago, four Ukrainian soldiers were killed by Russian separatist shelling in eastern Ukraine. Russian President Vladimir Putin acknowledged the increase in violence on Tuesday, and blamed Ukrainian forces for it. An official readout from the Kremlin of a phone call that Putin had with the French and German leaders said he had, “expressed serious concern about the escalation of armed confrontation on the contact line being provoked by Ukraine.” The U.S. Embassy in Kiev issued a statement last week calling on Russia to “observe ceasefire measures” and “end daily violence that is causing senseless suffering,” and end its aggression in Ukraine. “Forgotten War” 02:55 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.