Feb 20 - Oil prices rose on Friday, headed for their first weekly gain in three, on growing concerns a conflict may erupt between the U.S. and Iran, after Washington said Tehran will suffer if it does not agree a deal on its nuclear activity in a matter of days. Brent crude futures rose 25 cents, or 0.4 per cent, to $71.91, while U.S. West Texas Intermediate crude gained 31 cents, or 0.5 per cent, to $66.74 as of 0437 GMT. Prices settled at a six-month high on Thursday after U.S. President