The first U.S. troops reinforcing NATO allies in Eastern Europe amid a Russian military build-up on Ukraine‘s border arrived on Saturday at Rzeszow military base in southeastern Poland.

A small plane carrying what a Polish military source said were U.S. chain of command personnel landed in the morning at Rzeszow-Jasionka airport as preparations continued at the base, which is near Poland’s border with Ukraine.

TV footage showed temporary accommodation being prepared at the G2A Arena in the nearby town of Jasionka, while workers could be seen building a fence around the venue.

“As announced, the first elements of the brigade battle group from the 82nd Airborne Division of the United States Army have arrived in Poland,” a Polish military spokesman said.

On Wednesday, U.S. President Joe Biden ordered nearly 3,000 extra troops to Poland and Romania to shield Eastern Europe from a potential spillover from the Ukraine crisis.

About 1,700 service members, mainly paratroopers from the 82nd Airborne Division, will deploy from Fort Bragg, North Carolina, to Poland “over the next days”, U.S. Army sources have said.

According to the Pentagon, a Stryker squadron of around 1,000 U.S. service members based in the German town of Vilseck will be sent to Romania.

The first additional U.S. troops arrived in Germany on Friday.

Russia has denied plans to invade Ukraine but has deployed more than 100,000 troops near Ukraine‘s borders and says it could take unspecified military measures if its demands are not met, including a promise by NATO never to admit Kyiv.

The new plan goes above and beyond the 8,500 troops the Pentagon put on alert last month to deploy to Europe if needed. NATO defence ministers are expected to discuss further reinforcements at their next meeting on Feb. 16-17.