Turkey is pushing to capture the town of al-Bab, the last major Islamic State group stronghold in northern Syria. But are others welcoming the new advance in the war against the militants? Not quite.

By seizing the city, Turkey would plant its firmest foothold yet in Syria. That is already causing frictions with other players in the country's war.

Syrian President Bashar Assad's government opposes the Turkish incursion. His military's air defenses have threatened Turkish warplanes, and on Thursday, three Turkish troops were killed outside al-Bab in what the Turkish military said was a Syrian airstrike.

At the same time, Turkey's Syrian allies are clashing with Syrian Kurdish fighters, who are another ally of the United States in the war on IS and are currently leading an assault on the Islamic State group's de facto capital Raqqa.

Here is a look at why al-Bab is important for Ankara and how it can influence the balance of power in Syria.

WHY AL-BAB?

For Turkey, capturing al-Bab is key to preventing Syrian Kurds from connecting the stretches of territory they have captured along the border. A contiguous Kurdish-held area in Syria emboldens Turkey's own Kurdish rebels. Ankara views the Kurdish forces on both sides of the border as linked and labels them terrorists.

A victory would push IS further from Turkey's border and further squeeze the militants in the city of Raqqa.

President Recep Tayyip Erdogan said Turkey was forced to descend to al-Bab, 30 kilometers (19 miles) south of the Syria-Turkey border, "to prepare a region there that is free of terror." Home to at least 2.7 million Syrian refugees, Turkey is looking to establish a "safe-zone" inside Syria.

But Turkey's priority is the Kurds.

Erdogan has also vowed to take the nearby town of Manbij, which the Kurds captured from IS this summer after 10 weeks of grueling battles. That victory allowed the Kurdish forces to expand west of the Euphrates River — a line Ankara said they must not cross.

Al-Bab, Arabic for 'the door', would provide Turkey with a new leverage with its NATO ally, the United States, and strengthen Ankara's influence over Syrian rebels at a time when a new Trump administration in Washington could halt support.

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THE BATTLEFIELD

Al-Bab is sandwiched between three rival forces. Moving from the north and west, Turkish-backed fighters are less than one kilometer (half a mile) away. Kurds are moving in from the east. Syrian troops are stationed to the south.

The Islamic State group is ready, erecting a wall around the entire town and its countryside to the south, according to satellite imagery by U.S.-based firm TerraServer shared by intelligence analyst Roa Komar. He said fighting could be as heavy as the battle for Manbij.

Some 1,500-3,000 Syrian fighters backed by 300-600 Turkish troops are involved in the three-month-old Turkish incursion into northern Syria, known as Operation Euphrates Shield, according to a Western military official speaking on condition of anonymity in line with regulations. So far they have captured some 1,800 square kilometers (7,000 square miles), largely sparsely populated rural areas cleared by Turkish artillery and warplanes.

Turkey's Syrian allies comprise diverse factions often plagued by infighting, including the ultraconservative Ahrar al-Sham and the U.S.-backed al-Mutassem Brigade. Turkey's ground presence provides the factions with some protection from Russian or Syrian airstrikes, which have pounded rebels elsewhere, said al-Mutassem's chief, Mustafa Sejari, on his Twitter account.

At the same time, the Kurds vow to take al-Bab as well, though they are more likely to prove a distraction for Turkey's assault. Kurdish fighters and the Turkish-backed forces are already battling over control of an IS-held village between al-Bab and Manbij.

Meanwhile, Turkey is aggressively recruiting among rebels. One opposition faction recently evacuated from Damascus suburb of Darayya to rebel-held northern Idlib is mulling whether to join Turkey's operation.

After surviving a four-year government siege, Abu Jamal, leader of Islam Martyrs Brigades, said his estimated 700 fighters are joining other battles in northern Syria to gain new experience. "We have never fought Daesh before," he said, using the Arabic acronym for IS.

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HOW DOES THE U.S. FEEL ABOUT IT?

Nowhere is the U.S.'s often muddled Syria policy more tested than in al-Bab.

The U.S. has mainly been trying, with little success, to prevent fighting between its two allies, Turkey and the Kurds. After U.S. Chief of Staff Gen. Roger Dunford visited Ankara in early November, an American military liaison was sent to Turkey for closer coordination on anti-IS operations.

Part of that coordination could be over the offensive against Raqqa, said Noah Bonsay, of the Brussels-based International Crisis Group.

The offensive has been led by the Kurds, but Washington recognizes the need for a more local Sunni Arab force to eventually capture and control the city with its majority Arab population, Bonsay said. Ankara, which also said it wants to take part in the Raqqa operation, is in a position to help in that.

But while focusing on fighting IS, Washington has struggled in playing the bigger power game with Russia, Turkey and other players in Syria.

Turkey has grown closer to Russia even as it consolidates the opposition, including ultraconservatives, under its leadership.

"The U.S. is going to wake up in 2017 ... to find that Turkey has all the local influence and all the local leverage, and we have exactly none or precious little," said Jennifer Cafarella, a Syria analyst with the Washington-based Institute for the Study of War.

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THE VIEW FROM MOSCOW AND DAMASCUS

Despite Assad's opposition to the Turkish incursion, his ally Moscow appears to be tolerating it in order to further cultivate its warming ties with Ankara.

Russia and Turkey have been finding common ground on Syria. Russia's Vladimir Putin and Erdogan have met several times since August, after Ankara apologized for downing a Russian warplane a year ago. Russian and Turkish military chiefs met three times in as many months, and Russian media say Moscow is sharing military intelligence with Ankara.

With the better ties, Russia likely aims to exploit Turkey's strains with the West. Turkey, in turn, prevents Moscow from growing closer to the Kurds.

Russia could see advantages in the Turkish foray — for example, in the city of Aleppo, where Assad's Russian-backed forces are threatening to crush the eastern, rebel-held enclave. Ankara is unlikely to risk its rapprochement with Moscow by sending its allied Syrian factions to rescue the opposition part of the city.

Russia, however, must also deal with its ally Assad's opposition to Turkish influence in Syria.

Turkey had to briefly halt its airstrikes around al-Bab after Syrian air defenses locked on its warplanes last week. The strikes resumed this week, raising speculation Moscow mediated.

Thursday's Turkish casualties could be a sign Damascus views Turkey's advance so close to al-Bab as a step too far. Ankara's account of how the three soldiers died is disputed by Syrian activists, who said they were killed by an Islamic State suicide attack the day before.

Foreign Minister Walid al-Moallem said Damascus "cannot accept having one Turkish soldier remain" in Syria.

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Associated Press writers Suzan Fraser in Ankara, Vladimir Isachenkov in Moscow, and Lolita Baldor in Washington contributed to this report.