President Omar Hassan al-Bashir of Sudan visited South Sudan on Friday for the first time since the South broke away from Sudan in 2011, and President Salva Kiir of South Sudan said the two leaders agreed to resume trade across their border. The two countries agreed in March to resume pumping oil through pipelines from south to north and ease the tensions that had threatened to erupt into war. Mr. Bashir said Friday that he had ordered Sudan’s borders with South Sudan to be opened for traffic and that he hoped the two sides could begin to normalize relations. Mr. Kiir said the two leaders agreed to continue talks to resolve the conflicts over disputed regions along their volatile 1,200-mile border. But even as Mr. Bashir’s visit raised hopes of eased tension on Sudan’s southern border, violence flared up again in the western region of Darfur, forcing about 50,000 people to flee into Chad over the past week.