GAZIANTEP, Turkey — Kurdish fighters closed in Sunday on the Islamic State-controlled border town of Tal Abyad in northern Syria, cutting roads and supply lines that lead to the extremists’ de facto capital in the Syrian city of Raqqa.

The advance follows a determined Kurdish sweep through a vast expanse of extremist territory.

Backed by scores of U.S. airstrikes, the Kurdish fighters have wrested control of at least 250 towns and villages from Islamic State in Syria’s northeast since beginning the offensive in early May.

Advancing from two Kurdish-held areas –- east and west of Tal Abyad –- the Kurdish forces, known as the People’s Protection Units, have squeezed Islamic State into an ever-shrinking patch of turf. By late Sunday, they had reportedly entered portions of Tal Abyad from the east.

Islamic State, an Al Qaeda breakaway faction, holds sway in large swaths of Syria and neighboring Iraq. It has been the target of a U.S.-led aerial bombing campaign since last summer.

The group’s recent seizure of the cities of Ramadi in Iraq and Palmyra in Syria have demonstrated that it remains a potent force. It appears to be on the back foot in its fight with Kurdish militiamen in northern Syria, however.

Tal Abyad, known as Gire Sipi in Kurdish, is a strategic frontier town that Islamic State has held for about 17 months. It is the closest border town to Raqqa.

The loss of Tal Abyad would represent a significant defeat for Islamic State, which smuggles vast quantities of oil worth many millions of dollars into Turkey via the border. Tal Abyad has also served as the primary entry point for foreign fighters seeking to join Islamic State.

The Kurdish advance has raised the prospect of Kurdish control of a vast stretch of Syria’s border with Turkey.

Since Kurdish fighters forced Islamic State out of Kobani, another border town, this year, they have expanded their zones of control, driving the extremists from desert and village.

The Kurdish advance has drawn ire from Turkish President Recep Tayyip Erdogan, who accuses the West of supporting “terrorist groups.” The U.S.-led coalition has repeatedly struck extremist tactical units and destroyed their fighting positions during the Kurdish advance.

The Syrian Kurdish faction expanding its territory in northern Syria is a close ally of the Kurdistan Workers Party, or PKK, which seeks Kurdish autonomy and has waged a three-decade war against the Turkish state. The PKK is regarded as a terrorist group by Turkey and the United States.

The Kurdish thrust in northern Syria seems sure to raise tension in Turkey, where a Kurdish-focused political party, the Peoples’ Democratic Party, secured 13% of the vote in this month’s parliamentary elections. That resulted in a Kurdish-linked party entering parliament for the first time in the nation’s history.

About 13,500 refugees, predominantly ethnic Arabs and Turks, have fled the Kurdish advance in the last 11 days and entered Turkey, the Turkish Foreign Ministry said.

The Turks have now sealed the border and refused entry to several hundred displaced people on the Syrian side, while using water cannon and firing warning shots to disperse amassed crowds over the weekend, according to local news accounts.

Images of gun-toting Islamic State fighters forcing civilians back from the border and into Tal Abyad spread across social media late Saturday, raising fear that the group is planning to use residents as human shields.

The Kurds have meanwhile sought to assuage fears of ethnic cleansing of Arabs and other non-Kurds. In a statement posted on social media, the group’s spokesman, Redur Xelil, implored civilians to come to areas under Kurdish control.

“We assure them that we will guarantee their security,” Xelil said. “They can return to their villages and property when security returns.”