After two months, the U.S.-led aerial campaign in Iraq has hardly dented the core of the Islamic State group's territory. The extremist fighters have melted into urban areas when needed to elude the threat, and they have even succeeded in taking new territory from an Iraqi army that still buckles in the face of militants.

In neighboring Syria, days of airstrikes have been unable to stop militants on the verge of capturing a strategic town on the Turkish border.

The limited results show the central weakness of the campaign: There is only so much that can be done from the air to defeat an extremist force that has swept over much of Iraq and Syria. The Islamic State fighters have proven elusive and flexible, able to reorganize to minimize the blows. And more importantly, there are almost no allied forces on the ground able to capitalize on the airstrikes and wrest back territory from the militants.

The exception: Iraqi Kurdish fighters, the most effective forces in Iraq, have made some modest gains the past week.

That only highlights how others have proven unable to do the same. The Iraqi military is undermined by corruption and command problems. A new Iraqi government has being trying to woo support from more Sunni tribesmen, whose fighters are seen as vital against the Sunni extremists, but so far there has not been a flood of support. In Syria, rebels supported by Washington are in no position to move against the extremists, and Syria's Kurds are not as well armed as Iraq's.

The U.S. launched airstrikes in Iraq on Aug. 8 and in Syria on Sept. 23. Several European nations are participating in Iraq, but not in Syria, where the U.S. was joined by a coalition of Arab allies. U.S. officials have warned repeatedly that the campaign will be long — even years.

The Pentagon press secretary, Navy Rear Adm. John Kirby, contended last week that the strikes have hampered the militants. Before the strikes, he said, "they pretty much had free rein. They don't have that free rein anymore, because they know we're watching from the air. ... They have dispersed, whereas before they were more structurally cohesive in certain places."

PROGRESS IN THE NORTH

Most of the success for the air campaign has been in rural, open areas of northern Iraq. Last week, airstrikes paved the way for the Iraqi Kurdish fighters known as peshmerga to plow into a string of towns held by the extremists near the Syrian border: Mahmoudiyah, Rabia and Zumar. The Kurdish offensive is aiming for the town of Sinjar, and if they capture it, the Kurds would secure a main road in and out of Syria that is a militant supply line.

The early airstrikes also halted the extremists' advance toward the Kurdish capital of Irbil and broke the Islamic State group's grip on the strategic Mosul Dam, enabling peshmerga and Iraqi troops to recapture it. Strikes were also instrumental in breaking a siege of the northern town of Amirli, which the militants had surrounded.

But the warplanes have largely avoided Mosul, Iraq's second-largest city and the Islamic State group's biggest stronghold, or the nearby town of Tal Afar, apparently to avoid civilian casualties that would boost support for the group among the region's Sunnis. That has left the extremists a virtual free hand there, which is unlikely to change anytime soon. Last week, retired Gen. John Allen, the U.S. envoy to the anti-Islamic State coalition, said operations to retake Mosul will start "within a year."

LOSSES IN ANBAR

It was in Iraq's western province of Anbar that the Islamic State group made its first dramatic land-grab in January by capturing the city of Fallujah. The group found substantial support among the province's overwhelmingly Sunni population, which is largely conservative and deeply resentful of the Shiite-led government in Baghdad,

The extremists are still making gains. Last week, they seized the towns of Hit and neighboring Kubaisa, sending Iraqi soldiers fleeing and leaving a nearby military base with its stockpile of weapons at risk of capture. Government forces still control most of the provincial capital of Ramadi, but the city is vulnerable.

Also at risk is the predominantly Sunni town of Abu Ghraib on the western edge of the capital of Baghdad. The militants already have a significant presence in the town.

MOSUL AS A WATERSHED MOMENT

Iraqis often describe the battle against the Islamic State group in terms of before and after Mosul, alluding to the watershed moment when the Iraqi military collapsed in the face of the extremists' takeover of the city in June.

Many point to poor training and the leadership crisis within the military as a major source of its disintegration. Former Prime Minister Nouri al-Maliki, who begrudgingly relinquished power last month, inflamed sectarian tensions within Iraq's military, dismissing many qualified senior Sunni officers and replacing them with less-qualified Shiite officers loyal to him.

After Mosul fell, al-Maliki called upon volunteers to reinforce the humiliated military, and many Shiite militias quickly reported for duty. But with different leaders and divided loyalties among the militias, they were impossible to control.

Kurdish peshmerga have fared much better. They seized parts of the north the Kurds long coveted, claiming they were doing so to protect them from the militants. They have fought well, despite a lack of training and old weapons, mainly because they are far more unified and share a common goal — setting up an independent Kurdish state.

SETBACKS IN SYRIA

Coalition warplanes appear to have been unable to turn the tide in Syrian city of Kobani, which the militants have besieged for weeks, battling with its Kurdish defenders. Turkey's president warned Tuesday that Kobani is on the verge of falling.

The strikes around the city appear to have been more limited than, for example, the bombardment in August of Amirli, Iraq. In that case, Iraqi troops and Shiite militiamen were poised to swoop into the city after the strikes. In contrast, Kobani's Syrian Kurd defenders are poorly armed and are hampered by longtime tensions with neighboring Turkey, which resents the fighters' ties to Kurdish separatists in Turkey.

The airstrikes in Syria have largely targeted the Islamic State group's infrastructure across the broad northern and eastern regions the extremists hold. Warplanes have hit checkpoints, tanks, training camps and even one of the group's main headquarters in its de facto capital of Raqqa. But activists say Islamic State fighters left many of the bases before the strikes began, and the U.S. and its partners hit empty facilities. Heavy weapons were moved into protected areas.

Coalition strikes also hit oil facilities to try to cut off smuggling by the extremists. That hurt the group's income, although not significantly, said retired Gen. Richard Zahner, a former senior military intelligence official who helped manage intelligence collection in the Iraq war.

"I think they are well on track and very much following their ongoing strategy" of consolidating their gains in the Sunni heartland, he said of the militants.

WINNING HEARTS AND MINDS

Any lasting solution must be rooted in a genuine effort to win support among Syrians and Iraqis — particularly Sunnis.

Iraq's new government, led by Prime Minister Haider al-Abadi, immediately urged the army to stop shelling populated areas where civilians may be caught in the crossfire. The government also is looking to establish a national guard driven by local leadership and recruitment to include more Sunnis.

Winning over the Sunni tribes is an essential part of the solution. Ramadi has yet to fall in part because key Sunni tribes in the city have not allowed it to. The Jughaifi and al-Bunimer tribes have helped Iraqi special forces to protect the Haditha Dam in Anbar. In the battleground town of Dhuluiyah, a single tribe, al-Jabbouri, has been the sole resistance to an Islamic State takeover.

But there are many Sunni tribes that haven't been won over. They are looking for signs al-Abadi will bring more Sunnis into positions of power and address their feelings of disenfranchisement.

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EDITOR'S NOTE: Vivian Salama, the Associated Press bureau chief in Baghdad, has reported on the Middle East for more than a decade.

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Associated Press writers Ryan Lucas and Bassem Mroue in Beirut and Qassim Abdul-Zahra in Baghdad contributed to this report.