NAYMIYAH, Iraq -- Iraqi forces secured the southern edge of the Islamic State group stronghold of Fallujah on Sunday, two weeks after the start of an operation to recapture the city, the Iraqi special forces commander overseeing the operation said.

Iraqi special forces, also known as its counterterrorism forces, have secured the largely agricultural southern neighborhood of Naymiyah under cover of U.S.-led coalition airstrikes, Lt. Gen. Abdel Wahab al-Saadi said. Special forces are now poised to enter the main city, al-Saadi, commander of the Fallujah operation, said.

The Fallujah operation coincides with a twin offensive on Islamic State strongholds in neighboring Syria. Syrian Kurdish forces are advancing on Manbij, an Islamic State-held city controlling the supply route between the Turkish border and the town of Raqqa, the militants' capital. At the same time, Syrian government troops are advancing on Raqqa from the south.

The slow-moving Iraqi operation was announced in May. An array of troops including Iraqi military divisions, the federal police and the largely Shiite militia groups, known as the Popular Mobilization Forces, had cleared the Islamic State from the majority of Fallujah's suburbs.

On May 30, Iraq's special forces began pushing into the city center, but they have faced stiff resistance. Fallujah has been under Islamic State control for more than two years, and the militants have been able to erect complex defenses.

Tall dirt berms dot the dusty fields to the city's south. A single column of counterterrorism Humvees snaked up toward a row of low-lying houses that mark the beginning of the main city.

"VIBED! VIBED!" shouted an Iraqi air commander from a small mobile base on Fallujah's southern edge. Using an acronym for a car bomb, the Iraqi special forces officer called to Australian coalition forces over a hand-held radio. Moments later, a plume of white, then black smoke appeared on the horizon. Commanders at the scene said the explosion was the result of a coalition rocket destroying the incoming car bomb.

Al-Saadi said coalition air power in Fallujah has prevented car bombs, a popular and deadly Islamic State weapon, from inflicting casualties on his forces, but they have still succeeded in slowing progress.

"We are expecting many more," once inside the city's more urban neighborhoods, al-Saadi said.

Fallujah is one of the last strongholds of the Islamic State in Iraq. While the militants once held nearly a third of the country's territory, their grip has slipped to less than half that, according to Iraqi Prime Minister Haider al-Abadi. The Islamic State still controls patches of territory in northern and western Iraq, as well as its second largest city, Mosul.

Also Sunday, military sources said a mass grave containing the remains of about 400 people was found near Fallujah.

Iraqi forces uncovered the grave in the northwestern suburb of Saqlawiyah on Sunday, a day after the government said it captured the area from the Islamic State.

"Most of the remains belonged to military personnel who were detained and killed by the organization," a military official said, referring to the Islamic State.

There was no government confirmation.

Paranoia in ranks

The success of airstrikes that have killed prominent figures of the Islamic State group is having dire repercussions for other members of the extremist organization. The group has been taking drastic measures to silence suspected moles, according to Syrian opposition activists, Iraqi intelligence officials and an informant for the Iraqi government who worked within the group's ranks.

Dozens of Islamic State members have been killed by their own leadership in recent months. Others have disappeared into prisons and still more have fled, fearing they could be next as the group turns on itself in the hunt for moles.

The fear of informants has fueled paranoia among the militants' ranks. A mobile phone or Internet connection can raise suspicions. As a warning to others, the group has displayed the bodies of some suspected spies in public -- or used particularly gruesome methods, including reportedly dropping some into a vat of acid.

Islamic State "commanders don't dare come from Iraq to Syria because they are being liquidated" by airstrikes, said Bebars al-Talawy, an opposition activist in Syria who monitors the group.

Over the past months, American officials have said that the U.S. has killed a string of top Islamic State commanders, including its "minister of war" Omar al-Shishani, feared Iraqi militant Shaker Wuhayeb, also known as Abu Wahib, as well as a top finance official known by several names, including Haji Iman, Abu Alaa al-Afari or Abu Ali Al-Anbari.

In the northern Iraqi city of Mosul, the biggest city held by the Islamic State across its "caliphate" stretching across Syria and Iraq, a succession of militants who held the post of "wali," or governor, in the province have died in airstrikes. As a result, those appointed to governor posts have asked not to be identified, and they limit their movements, the Iraqi informant told The Associated Press. Iraqi intelligence officials allowed the AP to speak by phone with the informant, who spoke on condition of anonymity, fearing for his life.

Rami Abdurrahman, who heads the Britain-based Syrian Observatory for Human Rights, said some Islamic State fighters began feeding information to the coalition about targets and movements of the group's officials because they needed money after the extremist group sharply reduced salaries in the wake of coalition and Russian airstrikes on Islamic State-held oil facilities earlier this year. The damage and the loss of important supply routes into Turkey have reportedly hurt the group's financing.

"They have executed dozens of fighters on charges of giving information to the coalition or putting [GPS] chips in order for the aircraft to strike at a specific area," said Abdurrahman, referring to the Islamic State in Syria.

The militants have responded with methods of their own for rooting out spies, said the informant. For example, they have fed false information to a suspected mole about the movements of Islamic State leader Abu Bakr al-Baghdadi, and if an airstrike follows on the alleged location, they know the suspect is a spy, he said. They stop fighters in the street and inspect their mobile phones, sometimes making the fighter call any unusual numbers in front of them to see who they are.

After the killing of the finance official, seven or eight Islamic State officials in Mosul were taken into custody and have since disappeared, their fates unknown, said the informant.

"Daesh is now concentrating on how to find informers because they have lost commanders that are hard to replace," said a senior Iraqi intelligence official in Baghdad, using the Arabic acronym for the Islamic State group. "Now, any [Islamic State] commander has the right to kill a person whom they suspect is an informer for the coalition."

Another Iraqi intelligence official said at least 10 Islamic State fighters and security officials in Mosul were killed by the group in April on suspicion of giving information to the coalition because of various strikes in the city.

Mosul also saw one of the most brutal killings of suspected informants last month, when about a dozen fighters and civilians were drowned in a vat filled with acid, one senior Iraqi intelligence official said.

After the Tunisian militant Abu Hayjaa was killed on the road outside Raqqa on March 30, Islamic State leaders in Iraq sent Iraqi and Chechen security officials to investigate, according to Abdurrahman and al-Talawy, the Syria-based activist. Suspects were rounded up, taken to military bases around Raqqa, and the purge ensued. Within days, 21 Islamic State fighters were killed, including a senior commander from North Africa, Abdurrahman said.

Dozens more were taken back to Iraq for further questioning. Of those, 17 were killed and 32 were expelled from the group but allowed to live, Abdurrahman and al-Talawy said, both citing their contacts in the militant group. Among those taken to Iraq was the group's top security official for its Badiya "province," covering a part of central and eastern Syria. His fate remains unknown.

Non-Islamic State members are also often caught up in the hunt for spies. In the Tabqa area, near Raqqa, Islamic State fighters brought a civilian, Abdul-Hadi Issa, into the main square before dozens of onlookers and announced he was accused of spying. A masked militant then stabbed him in the heart and, with the knife still stuck in the man's chest, the fighter shot him in the head with a pistol.

Issa's body was hanged in the square with a large piece of paper on his chest proclaiming the crime and the punishment. The Islamic State circulated photos of the killing on social media.

Information for this article was contributed by Susannah George, Qassim Abdul-Zahra and Bassem Mroue of The Associated Press; and by staff members of Deutsche Presse-Agentur.

A Section on 06/06/2016