BEIRUT: Kurdish-led forces Monday advanced deep into territory in Syria held by ISIS, showing new momentum after they unexpectedly and swiftly seized a border crossing from the militants last week.

The Kurds, aided by U.S.-led airstrikes and smaller Syrian rebel groups, have pushed to within 7 km of Ain Issa, a town 50 km north of Raqqa, ISIS’ de facto capital, said Redur Xelil, spokesman for the Kurdish forces, and according to the Kurdish People’s Protection Units (YPG) took a former army base, Brigade 93.

The Syrian Observatory for Human Rights, an anti-regime activist group, said the Kurds had taken large areas of the base.

The rapid advance into Raqqa province has defied expectations of a protracted battle between the YPG and ISIS fighters, who waged a four-month battle for the border town of Ain al-Arab, where the Kurds finally defeated the jihadis in January.

Raqqa is the main seat of power in Syria for ISIS.

The U.S. has been leading an air campaign against ISIS in both countries since last year. The Kurds have been the most important partner so far for the U.S.-led campaign in Syria, where Washington has far fewer allies on the ground than in Iraq.

The Kurdish front in northern Syria has been one of the few sources of good news for the global campaign against ISIS since the jihadis made major advances last month in western Iraq and central Syria.

A spokesman for the Pentagon said last week ISIS forces had appeared to “crack” at the Turkish border town of Tal Abyad, which fell to the YPG in less than two days, cutting ISIS’ supply route from Turkey.

ISIS had held the former army base since capturing it from the Syrian military last year. If it falls completely to the Kurds, that would mean Ain Issa had effectively fallen, the observatory said. Thousands of people had fled from Ain Issa toward Raqqa city in the last two days, it added. The Kurdish advance is alarming the Turkish government, which is worried the growing Kurdish sway in northern Syria could inflame ethnic unrest among its own Kurdish population.

The Syrian Kurds say they do not want their own state, but see their example of regional autonomy as a model for how to settle the war in Syria and elsewhere in the region.

The Kurdish administration’s growing strength has led to friction with the Damascus government, which has tended to avoid direct conflict with the Kurds during the 4-year-old war while maintaining a foothold in areas where the Kurds hold sway.

Also Monday, Kurdish forces discovered a 400-meter tunnel dug by ISIS militants near the Turkish border with Syria, a spokesman for the militia said.

The YPG, discovered the half-completed tunnel after they captured the border town of Tal Abyad last week, spokesman Redur Khalil said. He added that it wasn’t clear whether it was eventually meant to lead into Turkey as the tunnel splits into two different directions at one point.

A tunnel linking Turkey to ISIS-held territory might offer some insight into the many ways in which Muslim radicals from around the world have slipped across the Turkish border to reach the fanatical group, whose territory straddles Syria and Iraq.

In Homs province, the army reopened a key oil supply route near ISIS-controlled Palmyra, as the jihadi group has mined the city’s ancient UNESCO-listed ruins.

However, the advance toward the central city from the west, reported by a pro-government newspaper and a monitoring group, and stepped up airstrikes do not indicate an imminent offensive to retake it.

ISIS overran Palmyra on May 21, sparking fears that the radical group might repeat the sort of vandalism it has carried out in Iraq and destroy one of Syria’s most famous archaeological sites.

“The infantry has made tangible progress in the area of Biyarat al-Gharbiyeh,” west of Palmyra, which had previously been in ISIS hands, said Al-Watan newspaper.

And the army had “intensified” its operations against ISIS in Palmyra and the surrounding area, the newspaper added.

The Observatory said the army was now 10 kilometers away from Palmyra. The monitor, which relies on a network of sources on the ground, said that President Bashar Assad’s troops had “chased ISIS fighters from Biyarat al-Gharbiyeh on the weekend.”

Observatory head Rami Abdel-Rahman said the capture of the area “allows the regime to secure a transport route for oil from the Jazal field ... to other Syrian cities under its control.”

Last week, government forces recaptured Jazal – one of the few oil fields still in government hands and about 20 kilometers northwest of Palmyra – from ISIS.

Abdel Rahman said the government depends on gas and oil fields around Palmyra to provide electricity to cities under its control, namely Damascus.

He said the army’s advances are just to protect these fields and that he doubted an offensive on Palmyra was imminent, noting that the government has no popular support in the city.

The army’s reinforcements to Biyarat al-Gharbiyeh came as Palmyra’s new city was hit by heavy regime raids in the past few days that killed 11 people, the Observatory said.

“The situation is very difficult for the city’s civilians because of the raids and the lack of water and electricity,” said a Palmyra militant who calls himself Mohammad Hasan al-Homsi.

The jihadis have laid mines in the spectacular Greco-Roman ruins of Palmyra, both the Observatory and Syria’s antiquities chief, Maamoun Abdulkarim, said Sunday.

It was not immediately clear whether the jihadis were planning on demolishing the ruins, or if they had laid the mines in order to prevent the advancing regime forces from retaking the city.

ISIS has regularly heavily mined seized territory to make any recapture more difficult.