The United States and Poland have signed an agreement updating an existing deal on basing U.S. missile interceptors on Polish soil.

U.S. Secretary of State Hilary Clinton attended the signing Saturday during a visit to the southern Polish city of Krakow.

With the agreement signed two years ago, the Bush administration had focused on protecting the U.S. against long-range intercontinental missiles with stationary interceptor rockets.

Since then, the U.S. has re-evaluated the threat from Iran, which Clinton says is developing short- and medium-range missiles capable of hitting Europe faster than previously thought.

Under the amended agreement, Poland will, starting in 2018, host a U.S. base, equipped with mobile interceptor rockets that can protect against missile attacks from any range.

Clinton expressed hope that Russia would drop its opposition to a U.S. missile defence system in Europe and accept an offer to co-operate in developing technologies for shooting down hostile weapons.

"The offer stands," she told a news conference after the signing, adding that the revised agreement would pose no threat to Russia.

Polish Foreign Minister Radek Sikorski said his country fully supports the amended plan.