The missile batteries, normally stationed in Germany, were repositioned at Poland's "invitation," the official said on condition of anonymity.

The move is seen as reflecting growing fear that a Russian missile could -- deliberately or not -- cross the border from neighboring Ukraine into NATO member Poland.

News of the deployment comes a day after Washington rebuffed a Polish offer to send Soviet-era MiG-29 fighter jets to Ukraine via a US air base in Germany, saying the proposal raised "serious concerns" for the entire NATO alliance.

US Vice President Kamala Harris was headed to Poland Wednesday to continue how to discuss how to best provide "military assistance" for Ukraine.

The Pentagon official said the Patriot missiles were sent to Poland as "a purely defensive deployment being conducted proactively to counter any potential threat to US and Allied forces in NATO territory."

He added that it was "100 percent in keeping with the seriousness with which we take our Article 5 commitments" to defend fellow members of the transatlantic alliance.

The official did not specify where in Poland the batteries were now based, nor their operational status -- saying only that they were "manned."

The Patriot surface-to-air missile defense system is capable of countering and destroying ballistic missiles, cruise missiles or aircraft.