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Approximately 1,650 service members from 15 different countries are participating in Exercise Rapid Trident 2017 which began Monday and will last until September 23, Pentagon spokesman Johnny Michael told CNN.

The U.S. involvement will include approximately 250 soldiers from the Oklahoma Army National Guard, 80 soldiers from the California Army National Guard, 45 airmen from the California Air National Guard, and some additional Army and Air Force personnel in support and control roles.

The annual exercise focuses on peacekeeping and stability operations and helps promote interoperability among the partner militaries of Ukraine and the United States as well as its NATO allies, according to U.S. Army Europe. It also serves to validate aspects of the Ukraine's military training program.

The exercise in Ukraine comes as Russia is gearing up for its major military exercise, Zapad 2017, which is expected to involve tens of thousands of troops operating along NATO's borders in Western Russia, the Russian European enclave of Kaliningrad and Belarus.

Military analysts see such war games as a possible rehearsal for a Russia-NATO armed conflict as it is taking place very close to an area that analysts consider to be the most likely target of a Russian offensive.

ZAPAD 2017 has also sparked concerns among NATO's easternmost members that Russian forces may stay in the area following the conclusion of the exercise.

Russian officials have said that only 13,000 troops will participate but western observers have said that as many as 100,000 Russian and Belorussian forces could be involved.

Read alsoNATO Rapid Trident 2017 war games kick off in UkraineNATO has been critical about how transparent Moscow has been about Zapad 2017, saying it has failed to adhere to international treaties by not allowing observers to monitor the exercise to ensure that it is not a cover for an aggressive military operation.

NATO's easternmost members have been wary of an increasingly aggressive Russia, particularly following Russia's 2014 military invasion and annexation of Crimea and Moscow's support of separatist rebels in eastern Ukraine.

"We urge Russia to share information regarding its exercises and operations in NATO's vicinity to clearly convey its intentions and minimize any misunderstandings," Pentagon spokesman U.S. Army Lt. Col. Michelle Baldanza told CNN.

"In response to this uncertainty, the U.S. has built a joint, persistent rotational presence of air, land, and sea presence in the region to support our Allies," she added.