The first unit of United States forces are expected to arrive in Poland on Saturday, defense sources told Reuters on Friday amid a Russian military buildup on Ukraine's border.

The forces are envisaged to reinforce NATO's eastern flank as the West and Russia wrangle over Russia's military presence near the Ukrainian border.

The sources, who asked for anonymity due to the sensitivity of the issue, said that the plans could change depending on security and logistics.

The U.S. military equipment has begun to arrive in Poland this week.

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.

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, while around 1,700 servicemembers, mainly from the 82nd Airborne Division, will be deployed from Fort Bragg, North Carolina, to Poland.

On top of that, 300 other service members are supposed to move from Fort Bragg to Germany.