ABUJA (Reuters) - Nigeria s President Muhammadu Buhari said the country  cannot afford to go back to dark days of insecurity  in the oil-producing Niger Delta region. In a speech to present the 2018 budget, Buhari called on all sides to maintain dialogue.  Last week, the militant group Niger Delta Avengers threatened a  brutal and bloody  return to the violence that last year slashed oil output. Oil minister Emmanuel Ibe Kachikwu will visit the region this week in an attempt to maintain the ceasefire. Buhari also said the country s exemption from production cuts organized by the Organization of the Petroleum Exporting Countries was a significant help to the country during its  most challenging time.  