Secretary of State Hillary Clinton, after meeting with Polish Foreign Minister Radoslaw Sikorski, told reporters in Poland the revised program will make the defense shield operational years sooner than a Bush administration plan.
"This agreement marks an important step in our countries' efforts to protect our NATO Allies from the threat posed by the proliferation of ballistic missiles and weapons of mass destruction," Clinton said.
Russia has objected to the plan, but Clinton said, "This is purely a defensive system. It's not directed at Russia. It is not a threat to Russia."
She also again invited Russian leaders to join in missile-defense efforts.
"Russia has not accepted that offer but the offer stands. And the United States is beginning discussions with Russia to explore whether there are any circumstances the United States and Russia could work together on radar development and deployment, or any other aspect of missile defense.
"We welcome that. We've encouraged that. Thus far there has not been a willingness by Russia to respond positively. But the door is open."
Notable deaths of 2014 [PHOTOS]