Marines patrolling this desert region near the Syrian border have for months been seeing a strange trend in the complex Iraqi insurgency. Insurgents, they say, have been fighting each other in this constellation of towns along the Euphrates, from Husayba to Qaim. The observations offer a new clue in the hidden world of the insurgency and suggest that there may have been, as American commanders suggest, a split between Islamic militants and local rebels.A United Nations official who served in Iraq last year and who consulted widely with militant groups said by telephone that there had been a split for some time. "There is a rift," said the official, who requested anonymity. "I'm certain that the nationalist Iraqi part of the insurgency is very much fed up with the jihadists' grabbing the headlines and carrying out the sort of violence that they don't want against innocent civilians."