In their Syrian strongholds, extremists from the Islamic State group had been moving into civilian apartment buildings for cover days before the U.S. and its allies began pounding them before dawn Tuesday, activists say. It's just one sign of the difficulties in trying to destroy the group by relying mainly on airstrikes.

Breaking the militants' hold over the cities they have captured in both Iraq and Syria will be complicated because the group can easily melt into the population. In the Iraqi city of Mosul, the extremists have enough support among the mainly Sunni Muslim population that they have reduced the presence of their fighters in the streets without apparent worry about their grip on power.

Another problem is that there are no allied forces on the ground poised to move in to control territory should the militants retreat under the aerial bombardment.

That's particularly the case in Syria, where rebels opposed to the Islamic State group have been almost completely driven from areas it controls. Across a broad stretch of eastern Syria, the only forces that could conceivably capitalize on the airstrikes at the moment are a few remaining units of President Bashar Assad's military, holed up in isolated bases in the Deir el-Zour and Hassakeh areas. But the Obama administration says it still wants Assad's ouster and doesn't want to help him regain ground.

So far, the coalition also has resisted calls by Kurds in Syria for arms, training and air cover. Those Kurdish forces, fighting in a group known as the YPG, had successfully pushed back the Islamic State group for two years in a band of territory that hugs the Syrian-Turkish border in the north and northwest. In recent days, however, the extremists have made gains in the area near the town of Kobani, forcing more than 130,000 people — mostly Kurds — to flee into Turkey.

A spokesman for the fighters said they could not match the firepower of the militants, who seized arms and armored vehicles from Iraqi forces fleeing their advance in June.

The U.S. and its allies have been carrying out airstrikes in Iraq for weeks, and Iraqi government forces, Shiite militiamen and Iraqi Kurdish fighters moved in to retake two sites in the north after bombardments pushed back the extremists: the Mosul dam and the besieged village of Amirli.

So far, the strikes have not targeted large urban areas such as Mosul, Fallujah and Tikrit, where breaking the extremists' grip is harder and the risk of civilians casualties is higher. In a sign of their confidence, Islamic State group fighters paraded 30 captured Iraqi soldiers in pickup trucks through the streets of Fallujah on Tuesday, only hours after the coalition strikes across the border in Syria.

The opening salvo in Syria by the U.S. and Arab nations blasted key government buildings held by Islamic State fighters in their de facto capital of Raqqa as well as the group's checkpoints and bases around the east. Syrian activist groups reported dozens of extremist fighters killed with seemingly few civilian casualties, although precise numbers were impossible to get.

But many of the buildings hit in the strikes were already empty, said Rami Abdurrahman, head of the London-based Syrian Observatory for Human Rights, which monitors the conflict through activists in Syria. Most of the deaths came in strikes on a training camp and on checkpoints outside cities, he said.

For days before, militant fighters in Raqqa melted away among the city's civilians, said Abu Ibrahim al-Raqqawi, who oversees another secret collective of activists. "They are taking apartments in civilian buildings, so you have six flats full of Daesh fighters, and four flats of ordinary people," he said, using the Arabic acronym for the Islamic State group.

Many of the militants sent their wives and children to nearby villages considered to be safer. Fighters have been appearing more rarely in the daytime and instead gather in public places like coffee shops at night, al-Raqqawi said, citing activists still in the city.

In Mosul — the largest Iraqi city in the territory across Syria and Iraq that is held by the group — residents say street patrols have been reduced because of the considerable support for the militants among the city's mainly Sunni Muslim population. There is deep resentment of the Shiite-dominated government in Baghdad and the Kurds, whose self-ruled area is nearby.

"When you're in a city, the population can be a screen for you — you can hide behind it," said Michael Knights, an Iraq expert with the Washington Institute for Near East Studies.

The problem comes if residents move away from the group or lose their fear of it, he said. "Then a highly populated city becomes a dangerous place; there are a million eyes, a million informants, a million people who can pick up an AK-47."

The governor of Ninevah province, where Mosul is the capital, is trying to organize a militia of Mosul residents against the Islamic State group. The official, Atheel al-Nujaifi, was among those who fled the city when the militants first overran it in June. Now he operates from an office in Irbil, the capital of the Kurdish autonomous zone.

A recent statement posted on al-Nujaifi's official website announced the plan for the so-called Mosul Battalions: "We will soon start the process of registering volunteers from Mosul to work toward liberating the city."

The Battalions claim to be operating already, announcing on social media there have been attacks on Islamic State group fighters, although it is impossible to confirm the statement's authenticity.

Mosul residents tell The Associated Press that a number of armed factions have formed in recent weeks, often made up of former soldiers seeking to avenge colleagues killed by the militants. The groups have used roadside bombs and guns with silencers in attacks on the extremists, according to the residents who spoke on condition of anonymity because they were concerned for their safety.

In Syria, the long-divided rebel factions are trying to position themselves to capitalize on any damage wreaked on the Islamic State group. Some have formed united "operation rooms," hoping to be a vehicle for aid the U.S. has promised to the rebels to fight the extremists. Most prominent among them is one called the "Euphrates Volcano," grouping moderate rebel factions and Syrian Kurds.

Rebels east of the Syrian capital of Damascus successfully pushed back Islamic State fighters from their villages and towns in the spring, said an activist who is part of a rebel group called the Brigades of the Habib Moustafa. The group is a more moderate faction that is also seeking aid and training from the West.

The activist, who spoke on condition of anonymity because he feared retaliation, said the rebels' strategy first involved uniting rebel groups in an area east of Damascus known as the Ghouta against the Islamic State group. The main resistance they encountered was in areas where residents supported the extremists. So the rebel factions launched a campaign to reduce support, distributing pamphlets that explained crimes committed by the jihadis and asking Muslim preachers to talk about atrocities in their sermons.

"We need to raise awareness to break the people's sympathy for them," the activist said.

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Hadid reported from Beirut. Sameer N. Yacoub and an Associated Press reporter in Mosul, Iraq, contributed to this report.