* Islamist violence has surged over past two months
* President ordered operation in May to crush rebellion
* Under pressure from party split and opposition coalition (Adds Boko Haram attack on Tuesday)
By Camillus Eboh and Joe Hemba
ABUJA/DAMATURU, Sept 18 (Reuters) - Nigeria's military said on Wednesday it had killed 150 insurgents, including a commander, in an operation against ...