(CNN) -

After three years of seesaw battles with the regime, Syrian rebels now face another daunting challenge: fending off radical Sunni militants who are taking over swaths of the country.

The Islamic State in Iraq and Syria (ISIS) has gained notoriety in recent weeks as the group captured city after city in Iraq. Its goal: To create a caliphate, or Islamic state, spanning Iraq and Syria.

Now, the crises in both countries are blending into a combined regional disaster as ISIS now controls land on both sides of the border -- opening the floodgates for weapons and fighters between Syria and Iraq.

All the cities between Deir Ezzor city and the Iraq border -- a stretch of 90 miles (150 kilometers) -- have fallen to ISIS, said Omar Abu Leila, a spokesman for the rebel Free Syrian Army.

Deir Ezzor itself is controlled partly by the government and partly by opposition militants, he said.

That's not all. ISIS also took over six Syrian oil and gas fields and a major pumping station that distributes oil from Iraq into Syria, Abu Leila said.

The captures include the al-Omar oil field, Syria's largest oil facility that can produce 75,000 barrels of oil a day. ISIS has also seized a military airport and a local army base.

The land grabs by ISIS now stretch from Syria's Deir Ezzor province to the group's recently gained territories in Iraq's Sunni heartland, the opposition Syrian Observatory for Human Rights (SOHR) said.

While ISIS has controlled parts of Syria for some time now, the group was fighting to open a route between its captures in Iraq and in Syria.

That victory came Thursday, when ISIS wrested control of the Albu Kamal border crossing and all the cities between the crossing and Deir Ezzor city from Syrian rebels.

Some Syrians turn to ISIS

ISIS has gained not just cities, but also some Syrians' claims of allegiance.

After capturing the town of al-Shahil, ISIS demanded last week that fighters surrender their weapons and repent for fighting ISIS.

The militants then called on residents to leave the town for a week to 10 days until "peace returns to the streets," according to a social media video obtained by activists.

In a video statement, leaders in the town of al-Shahil announced their withdrawal from all anti-ISIS groups.

A town representative announced by video that they "decided to swear allegiance" to shadowy ISIS leader Abu Bakr al-Baghdadi.

But about 30,000 people have fled the town, Abu Leila said.

Other Syrians flee

Days after their land grabs across Deir Ezzor province, ISIS militants have not let residents return to the towns of Khsham and Tabyeh, the SOHR said.

About 15,000 people lived in each town.