Russia’s most senior arms executive said Monday that a contract to supply Iran with powerful S-300 air defense missiles was now active. But with no delivery date or any other details, the announcement seemed aimed more at warding off an Iranian lawsuit than a major step toward delivery of the weapon system. Sergei V. Chemezov, Russia’s chief arms trade executive, made the announcement at an air show in Dubai, Russian news agencies reported. “The contract to deliver the S-300 was not only signed by both sides, but has already come into force,” Mr. Chemezov was quoted as saying by the RIA Novosti news agency. The initial contract to deliver the weapons was signed in 2007, before the current international arms embargo against Iran was established. But the deal was canceled in 2010 by Dmitri A. Medvedev, then Russia’s president, under pressure from the United States and Israel. In response, Iran filed a lawsuit claiming breach of contract that could cost Moscow as much as five times the $800 million value of the original deal. President Vladimir V. Putin reversed Mr. Medvedev’s decision in April, as a nuclear deal between Iran and the world powers began to appear likely. But the system still has not been delivered. The hitch, military experts say, is that Russia wants to keep the unfulfilled contract as a bargaining chip in its relations with the West, perhaps with an eye to ending economic sanctions imposed over its actions in Ukraine.