(CNN) -- The extremist Sunni militant group that recently declared a caliphate in parts of Syria and Iraq wrested control of a major Syrian oil field in a sweeping land grab Thursday, a UK-based monitoring group said.
A string of villages and towns along the Euphrates River fell like dominoes to the Islamic State in Iraq and Syria, bridging the Syrian province of Deir Ezzour with the group's recently gained territories in Iraq's Sunni heartland, the Syrian Observatory for Human Rights (SOHR) reported.
A video statement from the leaders in the town of al-Shahil announcing their withdrawal from all anti-ISIS organizations triggered the quick fall of a majority of Deir Ezzour province.
"We also decided to swear allegiance to the Caliph Abo Baker Al Baghdadi, noting that we are not responsible for those who breach this agreement and to call him to account if possible," a town representative in the video says in reference to ISIS's shadowy leader, Abu Bakr al-Baghdadi.
Fighters for the al Qaeda-linked Nusra Front, the largest and most powerful affront to ISIS, had been headquartered in al-Shahil. Fighters began melting away late Wednesday.
After taking control of al-Shahil, ISIS demanded that fighters surrender their weapons and repent for fighting ISIS. It then called on residents to leave the town for a week to 10 days until "peace returns to the streets," a social media video obtained by activists shows.
With Nusra Front's bastion in the east defeated, ISIS militants simply rolled through, unopposed, staking their black and white flag through most of the oil-rich province, the opposition Local Coordination Committees of Deir Ezzour said in a statement.
"Most of the activists that were opposed to ISIS have disappeared. Some are trying to flee to Turkey and others are pledging allegiance to the Islamic State along with other factions." Abu Abdallah, an opposition activist in Deir Ezzour, told CNN.
The capture of al-Omar oil field, the country's largest and most important oil facility, with a capacity to produce 75,000 barrels of oil daily, is the jewel in a string of gains that includes a military airport and a local army base.
"We took control without any clashes. They just fled," a bearded radical fighter from ISIS said in amateur video posted online Thursday from the oil field, "as proof, here is their ammunition and weapons."
Resource-poor rebel factions in Aleppo responded to the series of surrenders by calling for a full mobilization of all able-bodied men to fight ISIS's relentless march on opposition held territory in Syria.
"We demand that everyone, including civilians, report to the front lines so it will be a decisive war between us and them. Let's us show them what Aleppo is made of," a statement from Abu Tawfiq, a Tawheed Brigade military commander, reads. "We should never allow them to keep destroying our country and killing our people. Anyone who does not heed this call has no excuse."
ISIS, so radical that al Qaeda's central command disowned it earlier this year, now controls an estimated 20,000 non-contiguous square miles stretching from Syria's Aleppo Province to just 60 kilometers west of the Iraqi capital, according to the SOHR.
The group rules by a barbaric interpretation of Islamic law that includes the torture and imprisonment of opponents, the oppression of women and Christians, and even public beheadings and crucifixions
Last weekend, the group announced the creation of a so-called Islamic State or Caliphate that erases all state borders and make its leader the self-declared authority over the world's 1.5 billion Muslims.
While many members of the faith dismiss the claims as outlandish, analysts say the rapid rise of the fanatical force is sure to have a lasting impact on the region, particularly in weak Middle Eastern states such as Jordan.
CNN's Raja Razek in Beirut contributed to this report.