Fighters from the Islamic State group killed 270 soldiers, guards and staff when they captured a Syrian gas field earlier this week in the bloodiest clashes between the al-Qaeda splinter group and President Bashar al-Assad's forces, a monitoring group said.
The fighters seized the Shaer gas field in the desert east of the ancient site of Palmyra on Thursday.
The British-based Syrian Observatory for Human Rights had earlier reported 90 deaths, but upped the toll on Saturday.
The Observatory, which monitors violence in Syria through a network of sources in the country on both sides, quoted "trusted sources" as saying that the Islamic State had "killed and executed" 270 people during the assault.
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It said at least 40 fighters of the Islamic State, formerly known as the Islamic State of Iraq and the Levant, were killed in the offensive.
It was not immediately possible to verify the report. Syrian state media made no mention of the attack.
"Since the beginning of the year there have been clashes between the Islamic State and the regime in some areas, but these are the largest," the Observatory's director Rami Abdel Rahman said.
Gruesome footage apparently recorded by the fighters at the gas field and distributed online showed dozens of bodies, some of them mutilated, strewn across a desert landscape.
Homs governor Talal Barazi confirmed the attack, but did not give a death toll.
"The armed men were present in the area beforehand, but they have now expanded their area of control with this new operation," he told the AFP news agency on Thursday.
The Islamic State was once the Iraqi affiliate of al-Qaeda, but al-Qaeda disowned it in February after tensions mounted over its expansion into Syria.
The Islamic State, which proclaimed a "caliphate" straddling Syria and Iraq last month, has in recent weeks taken over the entire countryside of oil-rich Deir Az Zor province.
Deir Az Zor borders Homs province as well as Iraq, where the group has spearheaded a major offensive that has seen large swathes of territory fall out of the Iraqi government's control.