LONDON — President Barack Obama reassured Prime Minister David Cameron on Saturday that his frustration over the mammoth oil spill in the Gulf of Mexico is not an attack on Britain as the two leaders tried to soothe trans-Atlantic tensions over the disaster. Cameron's Downing St. office said the two leaders held a "warm and constructive" telephone conversation for more than 30 minutes. Obama has recently sharpened his criticism of BP PLC as the company struggles to stop millions of gallons of oil gushing from its ruptured deep-sea well.