Despite their strong show of unity, Trump disagreed with Duda's attempt to force the retirement of some Supreme Court judges, a decision last year that prompted protests.

"No," Trump said when asked if he supported the move.

Trump announced the troop deployment at a joint news conference with Duda in the White House Rose Garden on Wednesday, shortly after the two leaders signed a joint declaration affirming defence co-operation.

Duda, who is considering naming the US installation Fort Trump, said the new influx of troops was needed because of Moscow's past aggression against Poland and to help solidify his country's ties to the West.

"Russia again is showing its unkind, unfriendly imperial face," he said. "Russia is always looking out to take our territory."

Trump said earlier the troop contingent could come from the US force in Germany, which he has accused of paying too little for NATO's common defence.

The decision by Trump represents an effort to cater to the interests of Poland, a key NATO ally, while not overly antagonising Russian President Vladimir Putin, with whom Trump would like to have friendly relations.

He and Putin are to meet in Japan in two weeks.

"I hope that Poland is going to have a great relationship with Russia. I hope that we're going to have a great relationship with Russia," Trump said.

The US already has troops in Poland as part of a 2016 agreement with the NATO military alliance in response to Moscow's annexation of Crimea from Poland's eastern neighbour Ukraine in 2014.

Australian Associated Press