The U.S.-led coalition intensified its aerial bombardment of ISIS positions Thursday in the Syrian border town of Kobani as the extremist group fought street battles with Kurdish forces and reportedly rushed in reinforcements.

The battle for the town near the frontier with Turkey has emerged as a major early test for the air campaign aimed at rolling back and eventually destroying the extremist group. It has also strained ties between Washington and NATO-ally Turkey over the long-term U.S. strategy in Syria.

Ankara has called for the creation of a buffer zone inside Syria to secure the border, but the White House and Pentagon said Wednesday the U.S. is not considering that option. Such a zone would be costly and complex to enforce.

Columns of smoke rose over Kobani as warplanes buzzed overhead Thursday. Two strong explosions apparently from an airstrike echoed from the edge of the town, a cluster of low-slung concrete buildings nestled in rolling hills.

The crackle of gunfire and blasts could be heard on the Turkish side, where people watched the fighting unfold from a stretch of farmland.

The coalition airstrikes have forced some ISIS militants out of Kobani.

The U.S. Central Command said five airstrikes south of Kobani since Wednesday had destroyed an ISIS support building and two vehicles, and damaged a training camp. The strikes also hit two groups of ISIS fighters, it said in a statement.

"Indications are that Kurdish militia there continue to control most of the city and are holding out against ISIS," it said, using an acronym for the ISIS, which controls large parts of Syria and Iraq.

But the Pentagon has said the town may yet fall to the extremists because air power alone cannot prevent it.

Despite the airstrikes overnight and into the morning, the ISIS fighters managed to capture a police station in the east of Kobani, said the Britain-based Syrian Observatory for Human Rights. It said 11 ISIS fighters were killed, adding that Kurdish fighters captured four militants. The station was later struck by coalition jets.

The Observatory, which relies on activists across Syria, said Kurdish forces had surrounded the militants near the station and that heavy fighting was underway.

The Observatory said the militants had seized more than a third of Kobani, but Kurdish officials disputed that, saying their forces had recaptured several parts of the town.

"I can confirm that they don't control a third of the city. There is only a small part of Kobani under the control of Daesh," said local Kurdish official Idriss Nassan, using an Arabic acronym to refer to the ISIS.

Both Nassan and the Observatory said more than 20 airstrikes have been conducted in the area since Wednesday afternoon.

The fight for Kobani has brought Syria's civil war yet again to Turkey's doorstep, and the U.S. and its allies have pressed Ankara to take a more robust role in the coalition. Kurds have held massive demonstrations across Turkey in which they accuse the government, which has deployed its tanks just across the frontier, of doing nothing to save the town.

Responding to the criticism, Turkish Foreign Minister Mevlüt Çavusoğlu said it was unrealistic to expect Turkey to launch a ground war against the ISIS on its own.

Cavusoglu spoke at a news conference in Ankara with visiting NATO chief Jens Stoltenberg, who said there was no easy solution for Kobani.

"ISIS poses a grave threat to the Iraqi people, to the Syrian people, to the wider region, and to NATO nations," Stoltenberg said. "So it is important that the whole international community stays united in this long-term effort."

Cavusoglu said Turkey is prepared to play a bigger part once a deal is reached with the coalition. "Turkey will not hold back from carrying out its role," he said.

Turkish officials have said that while they do not want Kobani to fall, they will not take on a greater role until the coalition outlines a broader strategy that also weakens Syrian President Bashar Assad, who at the moment is best positioned to benefit from any rollback of the ISIS.

Ankara also is suspicious of the Syrian Kurdish forces fighting in Kobani, seeing them as an extension of the Kurdish PKK, which waged a long and bloody insurgency against Turkey.

The U.S. special envoy for the coalition, retired Marine Gen. John Allen, was expected to travel to Turkey after a stop-over Thursday in Egypt in which he met with Egyptian and Arab League officials. Secretary of State John Kerry and Turkish Prime Minister Ahmet Davutoğlu have spoken at least twice this week.

ISIS militants launched their offensive on Kobani in mid-September, capturing several nearby Kurdish villages and steadily strengthening their control around the town. The fighting has forced at least 200,000 residents to flee over the frontier into Turkey.

The Observatory's chief, Rami Abdurrahman, said more than 500 people have been killed in and around Kobani since the fighting began. He said the ISIS was rushing in reinforcements, indicating the extremists also view Kobani as a test of will.