Agence France-Presse: US and British leaders sought to ease tensions amid sharpened rhetoric over the expanding oil spill in the Gulf of Mexico, as President Barack Obama readied to take matters into his own hands. As political pressure mounts over his handling of the worst environmental disaster in US history, Obama will make his fourth visit to the disaster zone Monday with a two-day trip to Mississippi, Alabama and Florida. The US Coast Guard meanwhile piled pressure on BP giving the embattled …