* Attacks show offensive to quell Boko Haram so far failed
* Violence surging in Nigeria's troubled northeast
* Boko Haram the No. 1 threat to Africa's top oil producer (Adds details, witnesses, analyst, background)
BENISHEIK, Nigeria, Sept 20 (Reuters) - Islamist Boko Haram militants killed 159 people in two roadside attacks in northeast Nigeria this week, officials said, far more than was ...