US secretary of state Condoleezza Rice and Polish foreign minister Radek Sikorski signed the agreement for the base, which has angered Russia.

The formal signing came six days after the two countries agreed to a deal that will see 10 US interceptor missiles placed just 115 miles from Russia's western frontier.

The deal has sparked threats from Russia that Poland is making itself vulnerable to attack -- even a nuclear one.

The deal, and Russia's rhetoric, have further strained Moscow's ties with the West in the wake of its invasion of Georgia.