WASHINGTON - The United States could still block the delivery of F-35 stealth jets to Turkey if Ankara buys Russia’s S-400 air defense system, a senior official said Wednesday. US defense giant Lockheed Martin delivered an F-35 to Turkish officials in Texas last week, but Assistant Secretary of State Wess Mitchell warned the program could still be halted. The advanced jets will remain on US soil in Arizona while their Turkish pilots are trained, giving the State Department time to intervene, the top official told a Senate hearing.