As NATO refocuses on its eastern borders after Russia's annexation of Crimea, the United States is quietly deploying more troops to train special forces in former Soviet bloc states anxious about Moscow's intentions.

Major exercises began last month in Poland, Slovakia and the Baltic states of Estonia, Lithuania and Latvia involving several hundred personnel from U.S. special forces, the U.S. European Command (EUCOM) said in a statement.

Long-term plans include further training drills that will see about 100 U.S. elite troops remain on the ground at any one time in NATO states close to Russia, with teams working in several countries simultaneously, U.S. official said.