* Monitoring group says 90 pro-Assad forces killed
* Says 21 Islamic State fighters killed
(Updates death toll, adds bullet points, edits)
BEIRUT, July 17 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
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
"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
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)