MURSITPINAR, Turkey — 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 beleaguered Kurdish defenders.

Forces of the Islamic State have overrun between one-quarter and one-third of Kobani, said Esmat Sheikh, a Kurdish political leader in the city.

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

Meanwhile, the Turkish government 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,” Turkish Foreign Minister Mevlut Cavusoglu told a news conference in Ankara.

High-level U.S. and NATO 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 border with Syria — as leverage to bring the U.S. more directly into the more than three-year 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 the White House 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.

The Syrian government has accused Turkey of fomenting the mayhem inside Syria, providing funding, aid and logistical support to various militant groups, including the Islamic State and the 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 Turkish forces have not moved against the militants. Instead, Turkish authorities have focused on keeping Kurdish reinforcements and weapons from reaching Kobani via Turkish land.

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

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 in various Turkish towns and cities, 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.

Another reason for Turkey’s refusal to aid Kurdish militias 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.