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* Monitoring group says 90 pro-Assad forces killed
* Says 21 Islamic State fighters killed (Updates death toll, adds bullet points, edits)
BEIRUT, July 17 (Reuters) - Militant group Islamic State seized a Syrian gas field and killed at least 90 people on Thursday in one of the bloodiest clashes between the al Qaeda offshoot and President Bashar al-Assad's forces, a monitoring group said.
Islamic State has made rapid gains in Syria in recent weeks, mostly by seizing territory from rival rebel groups, using weaponry brought in from Iraq where last month it managed to take large areas from government forces.
Activists say the Syrian air force has in recent weeks stepped up attacks on positions held by Islamic State, formerly known as the Islamic State in Iraq and the Levant.
On Thursday morning, the group launched an attack against the Sha'ar gas field east of Homs, killing at least 90 of the men guarding it in a "wide assault" from several directions, the Britain-based Syrian Observatory for Human Rights said. It said 21 Islamic State fighters were killed in the offensive.
The Observatory, which monitors violence in Syria through a network of sources in the country, quoted "trusted sources" as saying a further 270 guards, government forces and militia members loyal to Assad were missing, taken prisoner, wounded or killed.
"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 Abdurrahman said.
Previous large clashes between government forces and Islamic State usually involved other rebel groups, he said. The two sides have also skirmished continuously in other areas but casualties had been relatively light, Abdurrahman said.
It was not immediately possible to verify the report. Syrian state media made no mention of the attack.
About 30 people had managed to escape to the nearby Hajjar field, the Observatory report added.
Islamic State has previously taken control of oilfields in Iraq as well as in Syria's eastern Deir al-Zor province. The group was once the Iraqi affiliate of al Qaeda, but al Qaeda disowned it in February after tensions mounted over its expansion into Syria.
Islamic State has declared a "caliphate" in the areas where it operates in Iraq and Syria, which include the Syrian city of Raqqa as well as Iraq's Mosul.
The Observatory says more than 170,000 people have been killed in Syria's conflict, which started as a peaceful protest movement in 2011 but descended into a multifaceted civil war after a government crackdown. (Reporting by Alexander Dziadosz and Oliver Holmes; Editing by Tom Perry and Robin Pomeroy)