Russia and the United States could still reach agreement on the missile defense issue but time is running out, Deputy Foreign Minister Sergei Ryabkov said on Friday.

“When the implementation of the third and then fourth and subsequent stages of the phased adaptive approach on the [U.S.] global defense system begins, the situation could alter for us,” Sergei Ryabkov said.

He reiterated Moscow’s demand for legally binding guarantees that U.S. and NATO missiles will not be aimed at Russia, warning that otherwise unspecified “compensatory” countermeasures would follow.

NATO Deputy Secretary General Alexander Vershbow indicated on Thursday that a compromise with Moscow on the U.S. missile defense system in Europe is possible, despite the current deadlock.

Several prominent Russian researchers and military experts have recently spoken out in support of NATO’s claims that the alliance’s plans do not target Russia, Vershbow noted, adding that their proposals could help Moscow and Brussels find common ground on the issue.