STOCKHOLM (AP) — Sweden has started its largest military exercise in over 20 years with nearly 20,000 troops drilling on air, land and sea, including a contingent of over 1,000 U.S. soldiers, amid rising military activity in the Baltic Sea region.

The three-week Aurora 17 drill kicked off Monday and will chiefly take place around the strategic Baltic Sea island of Gotland and the regions surrounding Stockholm and Goteborg.

The Swedish military said the exercise by the non-NATO nation is designed “to deter potential attackers, and force them to carefully consider the risks of attacking our country.”

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Troops from non-NATO Finland will also attend along with NATO soldiers from Denmark, Norway, France, Estonia, Lithuania and the United States.

Russia and its neighbor Belarus are holding a massive joint-military exercise, Zapad 2017, that begins Thursday near the borders of Estonia, Latvia and Lithuania.