KARABILA, Iraq, June 20 - Late Sunday night, American marines watching the skyline from their second-story perch in an abandoned house here saw a curious thing: in the distance, mortar and gunfire popped, but the volleys did not seem to be aimed at them.
In the dark, one spoke in hushed code words on a radio, and after a minute found the answer.
"Red on red," he said, using a military term for enemy-on-enemy fire.
Marines patrolling this desert region near the Syrian border have for months been seeing a strange new trend in the already complex Iraqi insurgency. Insurgents, they say, have been fighting each other in towns along the Euphrates from Husayba, on the border, to Qaim, farther west. 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 in a telephone interview that there has been a split for some time.
"There is a rift," said the official, who requested anonymity, citing the sensitivity of the talks he had held. "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."
The nationalist insurgent groups, "are giving a lot of signals implying that there should be a settlement with the Americans," while the Jihadists have a purely ideological agenda, he added.
The insurgency is largely hidden, making such trends difficult to discern. But marines in this western outpost have noticed a change. For Matthew Orth, a Marine sniper, the difference came this spring, when his unit was conducting an operation in Husayba. Mortar shells flew over the unit, hitting a different target.
"The thought was, "They're coming for us. But then we saw they were fighting each other," he recalled during a break in Monday's operation. "We were kind of wondering what happened. We were getting mortared twice a day, and then all of a sudden it stopped."
Access for the foreign fighters is easy through the porous border with Syria, where the main crossing, Husayba, has been closed for seven months to stem their flow. "They will come from wherever we are not," said Col. Stephen Davis, the commander of the Second Regimental Combat Team of the Second Marine Division. "Clearly there are foreign fighters here and quite clearly they are coming in from Syria."
Marines have conducted several offensives in villages along the Euphrates, including one over the past few days in Karabila, to disrupt the fighters' networks. During raids on mostly empty homes, they found nine foreign passports, and of about 40 insurgents killed, at least three were foreign, marines said.
Capt. Chris Ieva, a fast-talking 31-year-old from North Brunswick, N.J., said he could tell whether an area was controlled by foreign insurgents or locals by whether families had cellphones or guns, which foreign fighters do not allow local residents to have for fear they would spy on them. Marines cited other tactics as being commonly employed by foreigners. Sophisticated body armor, for example, is one sign, as well as land mines that are a cut above average, remote-controlled local mines, and well-chosen sniper positions.
When the marines were fighting in an operation in the area in early May, five marines were killed after their tank rolled over a mine that had been set for vehicles with large distances between the treads.
In Karabila, marines picked their way through empty houses over the past four days, looking in closets and behind closed doors, into the hidden lives of insurgents who had left behind caches of weapons, medical supplies and Jihadist literature, including an inspirational guide that attempted to justify beheading by using Islamic scripture.
As the operation ended about 6 p.m. Monday, marines, successful in their mission, lined the roof of the last house they took against the backdrop of plumes of smoke. Captain Ieva said: "Will some come back? Yes. But the bigger fruit is disrupting them. We've made them uncomfortable in their own system."