Financial losses over a protracted blockade of Libya’s vital oil fields and ports have rapidly swelled, surpassing $2 billion on Tuesday, the country’s National Oil Corporation said. Oil supplies have become a major point of conflict between rival governments in the war-torn country. Powerful tribes loyal to Libya’s eastern-based forces seized large export terminals and choked off major pipelines last month, aiming to starve the U.N.-backed government of crucial revenues.