The U.S. is expected to announce Wednesday that it will send about 1,000 additional troops and a squadron of Reaper drones to Poland to beef up the nation's ability to defend itself amid worries about Russian military activity, U.S. officials said.

The final details were still under discussion Tuesday, but the decision to expand America's military presence in Poland comes after months of lobbying by Polish leaders who had hoped for a U.S. base in their country.

Officials said Tuesday that the preliminary agreement avoids any permanent U.S. base or presence in the country and sticks instead to a rotational force. There are currently about 4,500 U.S. troops that routinely rotate in and out of Poland.

The new plans call for the construction of a new combat training center in Drawsko Pomorskie and additional facilities in the future. U.S. officials said the Reaper drones will be used to provide greater intelligence to Poland. The U.S. also expects to set up a military headquarters there.

An announcement is expected Wednesday when President Donald Trump meets with Polish President Andrzej Duda at the White House. An aide to Duda has said that negotiations to increase American troops in Poland were successful.

Earlier this year Polish leaders said they ideally would like a permanent U.S. base and said it could be called "Fort Trump." But beefing up the current U.S. military rotations was always seen as a more realistic move.

Following the Russian annexation of Ukraine's Crimean Peninsula in 2014, the U.S. has again been increasing military activity in Europe in concert with NATO allies. That includes stationing four multinational battalion-size battlegroups in four eastern nations of the alliance, Poland, Estonia, Latvia and Lithuania, led respectively by the U.S., Britain, Canada and Germany.

The eastern European nations have reached out to the U.S. and NATO for greater protection, worrying that they might be the next target of Russia's military advance.

The increase in U.S. forces in that region also reflects America's new national defense strategy that declares great-power competition with China and Russia as a top priority.

A senior administration official on Tuesday said the U.S. wants to underscore its commitment to the defense of the region, and said Poland is a key partner. A second official declined to discuss details of the announcement but called it a new facet of the two nations' military-to-military relationship that will enhance deterrence in Europe.

The officials spoke on condition of anonymity to discuss the matter ahead of the president's meeting Wednesday.