The U.S. military is due to send troops to Romania and Bulgaria during the summer as part of an operation to counter Russian hostility in Europe, but U.S. officials now say the operation may be expanded to include Hungary, the Czech Republic and Georgia. Operation Atlantic Resolve, which already has U.S. troops training in Poland, Estonia, Latvia and Lithuania since April 2014, is aimed at demonstrating U.S. commitment to Europe and NATO “in light of the Russian intervention in Ukraine,” says a U.S. Department of Defense mission statement.

According to Col. Michael Foster of the 173rd Airborne Brigade, based in Vicenza, Italy, the operation is expected to expand into northern and southern regions of Europe.

"So by the end of the summer, you could very well see an operation that stretches from the Baltics all the way down to the Black Sea," Foster said, speaking at the Center for Strategic and International Studies in Washington on Monday. "As you connect countries, there is almost a line of U.S. troops.”

Around 300 U.S. troops  from the 173rd Brigade are being sent to Ukraine to help train troops from the country’s National Guard, which is a separate mission from Atlantic Resolve, "but certainly tied in to deterring Russian aggression,” said Foster.

The training comes at the request of the Ukrainian government and will focus on military skills, from basic first aid to ambush operations. Come October, Ukrainian units with on-the-ground experience fighting pro-Russian rebels will be paired with U.S. military units.

"What we have been asked to do, and the issue that exists in the fight, is not an issue of teaching the Ukrainians things they don't know but refining their processes and pushing them forward," Foster said. "The fight from my perspective only is, it's not an imbalance of capability, it’s an imbalance of capacity in some places, or a refinement in one area that's giving Russia tactical overmatch in a few specific areas."

The inclusion of Bulgaria and Romania into the operations brings the total number of U.S. troops involved in Operation Atlantic Resolve to around 1,900.