* U.S. offers written pledge system won't threaten Moscow
* Chance for Russian involvement not "infinite," U.S. says
* U.S. would let Moscow use own sensors to measure test
WASHINGTON, Oct 18 (Reuters) - The United States has invited Russia to use its own radars and other sensors to size up one or more U.S. missile-defense flight tests as part of a new push to persuade Moscow that the system poses it no threat, a Pentagon official said on Tuesday.
The idea is to let Russia measure for itself the performance of U.S. interceptor missiles being deployed in and around Europe in what Washington says is a layered shield against missiles that could be fired by countries like Iran.
"These are smaller missiles," Army Lieutenant General Patrick O'Reilly, director of the Pentagon's Missile Defense Agency, told a forum hosted by the Atlantic Council. He referred to current and planned Standard Missile-3 interceptors built by Raytheon Co .
They would be ineffective as anti-missile interceptors against a country like Russia, whose strategic deterrent missiles are launched from deep inside its territory, he said. The SM-3 interceptor, to be based on land and at sea, "can't reach that far."
President Barack Obama pleased the Kremlin in 2009 by scrapping his predecessor's plan for longer-range interceptor missiles in Poland and a radar installation in the Czech Republic, a move that helped to improve U.S.-Russian ties.
But Moscow says that Obama's revised version, which includes participation by Romania, Poland, Turkey and Spain, could undermine Russia's security if it becomes capable of neutralizing Russia's nuclear deterrent and has warned of a new arms race if its concerns are not met.
Ellen Tauscher, undersecretary of state for arms control and international security, told the forum that the United States was prepared to offer Moscow written assurances that the system being built is not directed against Moscow.
But Tauscher, who held talks in Moscow last week on the issue with Russian Deputy Foreign Minister Sergei Ryabkov, added: "We cannot provide legally binding commitments, nor can we agree to limitations on missile defense, which must necessarily keep pace with the evolution of the threat."
She said she could not predict whether Russia and NATO would reach an agreement on missile defense cooperation in time for a NATO alliance summit next May that is due to consider the system's progress. The United States would like to partner with Moscow to boost its performance, including by using Russian radar systems.
"As time goes on it gets harder (to wrap in Russia)," Tauscher said, "because the aperture to join this system will close eventually. It's not an infinite opportunity."
The Missile Defense Agency, in a follow-up email to Reuters, said it had not yet determined which test or tests it would open to active Russian participation.
Russia would not receive any classified performance data on the U.S. system, said Richard Lehner, an MDA spokesman, but would be welcome to use its own radars, sensors and other know-how to measure interceptor speed, altitude, distance and other parameters.
Tauscher said the planned missile shield would be robust enough to manage the threats that Washington projects in the Middle East but "certainly would only chase the tail of a Russian ICBM or SLBM." Those are the acronyms for long-range missiles fired from land or from submarines.
"And that's the truth," she said. "Perhaps only with their eyes and ears will Russians embrace that."