MOSCOW - Russia is preparing three naval vessels for a long-term mission to the Persian Gulf within the next month to protect its 'national interests' in the event of an American invasion of Iraq. The move could heighten tension between Moscow and Washington - both having interests in Iraq's oilfields - and comes at a time when the United States is doubling the size of its troop deployments to the Gulf region. Advertisement Sources in the eastern port of Vladivostok said yesterday that the anti-submarine ships Admiral Panteleyev and Marshal Shapo- shnikov - which are armed with missiles and reconnaissance...