Russia still wants legal guarantees it is not targeted by a U.S.-led missile shield, despite an invitation to observe interceptor test launches in Europe, the Russian Foreign Ministry said on Wednesday.

“The hypothetical presence at test launches may be regarded only as a measure of transparency and trust,” the ministry said.

“Such measures are undoubtedly useful but they do not eliminate the need to receive clear, legally binding guarantees that the European segment of the U.S. global missile defense is not directed against Russia’s strategic potential,” it said.

Russia and NATO agreed to cooperate on the so-called European missile defense system at the Lisbon Summit in November 2010. NATO insists there should be two independent systems that exchange information, while Russia favors a joint system with full-scale interoperability.

Washington has invited Russia to use its own radars to measure the test-flight of one or more U.S. interceptor missiles.

Russia has agreed to cooperate on the new system, but warned it may strengthen its own defenses if it is not made a full partner in the U.S.-led shield.