The Islamic State of Iraq and Syria (ISIS) is now in control of 35 percent of the Syrian territory following a string of victories, the London-based Syrian Observatory for Human Rights said Friday.
ISIS holdings include nearly all of Syria's oil and gas fields, the monitoring group said in a statement, Al Arabiya reported.
One of the latest gains of the self-proclaimed "caliphate" was the seizure of the country's biggest oil fields, in Deir el-Zour in eastern Syria, earlier in the week.
Deir Ezzor borders Homs province as well as Iraq, where the jihadist group has spearheaded a major Sunni militant offensive that has seen large swathes of territory fall out of the Baghdad government's control.
Meanwhile, jihadists have killed 270 Syrian regime fighters, civilian security guards and employees since seizing a gas field in Homs province, the Observatory added, according to Agence France-Presse.
The London-based group described Thursday's takeover of the Shaar field as "the biggest" anti-regime operation by the ISIS since it emerged in the Syrian conflict a year ago.
The watchdog said it had documented "the death of 270 people killed in the fighting or executed" since Thursday.
"A large majority of the men killed were executed at gunpoint after being taken prisoner following the takeover of the camp," Rami Abdel Rahman, the Observatory director said.
"Eleven of the dead were civilian employees, while the rest were security guards and National Defence Forces members," he added.
A counter-attack by Bashar al-Assad's forces on Friday left 40 ISIS militants dead, Abdel Rahman said.
The fate of nearly 100 people who worked at the site remained unknown, according to earlier figures released by the Observatory.
Abdel Rahman condemned the deaths of the 270 Syrian regime fighters.
"The Observatory condemns summary execution as a war crime, regardless of which side it is committed by in the Syrian conflict," he said.
"Summary execution is a war crime -- whether of civilians or combatants. They are prisoners of war and must not be executed."
The Syrian government did not officially confirm the deaths, but supporters of Assad posted photographs of the dead, and described their killings as a "massacre".
One pro-regime Twitter user wrote: "Thirty martyrs were brought to Homs hospital from the Shaar gas field... Homs is still bleeding."
Gruesome footage apparently recorded by the jihadists at the gas field and posted on YouTube showed dozens of bodies, some of them mutilated, strewn across a desert landscape.
One video shows a jihadist posing with the bodies as he speaks in German interspersed with religious terms in Arabic, seemingly celebrating the killings