Oil prices may have further to fall and a rebound could take some time, despite increasing signs that the downtrend will end, possibly in the second half of this year as North American supply growth slows, the West’s energy watchdog said on Friday. Crude oil prices have fallen almost 60 percent over the last six months with both of the world’s crude oil benchmarks now trading below $50 a barrel as supplies of high quality, light oil from the United States and Canada have overwhelmed demand at a time of lacklustre global economic growth.