MOSCOW (Reuters) - The Russian Foreign Ministry on Thursday told the United States to step back from a confrontation over military observation flights before the two countries became embroiled in another round of tit-for-tat retaliatory measures. The United States has accused Russia of flouting the so-called Open Skies Treaty, an agreement designed to build confidence between the two countries  militaries, and said it plans to take measures against Moscow. The Wall Street Journal newspaper reported on Tuesday that would include restricting Russian military flights over American territory in response to what it said was Moscow preventing U.S. observation flights over its heavily militarised Baltic exclave of Kaliningrad. Russian Foreign Ministry Spokeswoman Maria Zakharova told a news briefing on Thursday that Moscow would respond to any new U.S. restrictions.   Nobody has canceled the principle of reciprocity in international relations,  said Zakharova.        