Related story: Airstrikes, street battles rock the Syrian city of Kobani

Explosions from apparent U.S.-led airstrikes rocked the besieged Syrian city of Kobani again Thursday, amid reports of intense street battles between advancing Islamist militants and the town’s Kurdish defenders.

Extremists with the group Islamic State have overrun between one-quarter and one-third of Kobani, said Esmat Sheik, a Kurdish political leader in the city.

“We’re here resisting without weapons and will do so till the last man,” said Sheik, reached on his cellphone in the city just south of the Turkish border.

Meanwhile, Turkey appeared to rule out a unilateral ground operation against Islamic State fighters.

“It is not realistic to expect Turkey to conduct a ground operation on its own,” Foreign Minister Mevlut Cavusoglu said at a news conference in Ankara, the capital.

High-level U.S. and North Atlantic Treaty Orgnaization officials were arriving in Ankara for talks this week as Washington and its allies seek enhanced participation against the militants from Turkey, a NATO member.

But Turkish authorities appear to be using their front-line position — Turkey shares a more than 500-mile-long border with Syria — as leverage to bring the United States more directly into the civil war against the government of Syrian President Bashar Assad.

The Obama administration has called for Assad to step down and has backed anti-Assad rebels. But it insists that the battle against Islamic State militants — who are also fighting Assad’s government — is the more pressing issue.

Turkish authorities, however, view toppling Assad as the priority.

“Assad’s regime is the cause of instability and therefore a political change is necessary,” Cavusoglu said.

Assad’s government has accused Turkey of fomenting the mayhem in Syria, providing funding, aid and logistical support to various militant groups, including Islamic State and Al Nusra Front, the official Al Qaeda franchise in Syria.

Last week, Vice President Joe Biden caused a furor when he told a Harvard forum that Turkish President Recep Tayyip Erdogan had acknowledged privately that Ankara had inadvertently fueled the militant rise, allowing too many Syria-bound militants to traverse Turkish territory. Erdogan denied admitting any mistake. Biden later apologized.

Turkey, with one of the region’s largest armies, has tanks and troops stationed just across the border from Kobani. But its forces have not moved against the militants. Instead, authorities have focused on keeping Kurdish reinforcements and weapons from reaching Kobani via Turkish land.

The militants surrounding Kobani on three sides have cut the city off from supply routes in Syria. Reinforcements from allied, Kurdish-controlled areas in northern Syria cannot help because they must pass through extremist-held territory to reach the city.

Ankara’s refusal to allow military aid to reach Kobani has triggered outrage among ethnic Kurds and led this week to violent demonstrations across Turkey. At least 25 people were killed in clashes, marking some of the nation’s most violent civil unrest in years, local media reported. Authorities imposed curfews and brought in security reinforcements to restore order.

Erdogan has charged that the protests were aimed at “sabotaging” the peace process between the Turkish state and the outlawed Kurdistan Workers’ Party, or PKK. The PKK and Ankara have been adversaries in a three-decade conflict that has left tens of thousands dead.

One reason for Turkey’s refusal to aid the Kurds in Kobani is that the town’s defenders are allies of the PKK, which Ankara lists as a terrorist group. Ankara has been extremely alarmed at the establishment of pro-PKK, semiautonomous Kurdish zones in Kobani and elsewhere along the Turkish border in northern Syria. The Kurdish-controlled regions emerged after Syrian government forces largely withdrew from the region more than two years ago.

In Kobani, militants appear to be advancing despite the accelerated pace of U.S-led airstrikes, including what appeared to be several new bombardments Thursday. Onlookers on the Turkish side of the border could see massive explosions on the western flank of Kobani, known in Arabic as Ayn al-Arab.

The Pentagon on Thursday confirmed a total of nine U.S. airstrikes to the north and south of Kobani, destroying or damaging Islamic State buildings, a tank, a heavy machine gun and fighting units.

The city’s Kurdish defenders, reportedly numbering several thousand, say the airstrikes have slowed the militant advance but will not be enough to halt Islamic State, which is heavily armed with weapons and armed vehicles seized in Syria and neighboring Iraq. The utility of air power is likely to diminish as the militants advance into urban terrain, where buildings provide cover and the positions of targets are difficult to pinpoint.

Most civilians have fled Kobani, once home to some 400,000, most of them ethnic Kurds.

Along the border, exiled residents seemed pessimistic that their fellow Kurds can hold out much longer against the militant assault.

“Maybe they can survive another 10 days,” said Mohammed Ali, 31, while standing on a hill watching the columns of smoke rise from Kobani. “The airstrikes are helping, but I fear they are too late.”

Johnson is a special correspondent. Times staff writer McDonnell reported from Beirut. Special correspondent Nabih Bulos in Beirut contributed to this report.

Twitter: @mcdneville