BEIRUT, Lebanon -- The Syrian insurgency claimed on Thursday to have near-total control of a strategically important province in the country's northeast, home to some of the few remaining domestic oil production facilities that supply fuel for President Bashar al-Assad's military forces, after ferocious clashes that lasted for three days.
The rebel assertions about the province, Hasaka, would, if confirmed, be at least the third significant gain by the insurgency this week, following the seizure of Syria's largest hydropower dam and the takeover of a northern military air base with much of its fleet still intact. Hasaka Province includes the ethnically mixed city of Shadadah, one of the 10 largest cities in the country.
In addition, rebels claimed to have shot down three Syrian Air Force warplanes on Thursday, corroborating their assertions with videos posted on the Internet. If so, that would be the government's biggest one-day loss of warplanes to insurgent fire in the conflict, which began nearly two years ago as a peaceful protest to Mr. Assad's autocratic rule and has cost what the United Nations has estimated to be nearly 70,000 lives.
The Local Coordination Committees, a network of anti-Assad activists in Syria, said gunners of the Free Syrian Army had felled two warplanes in Idlib Province in the north and one in central Hama Province.
Hasaka, about 375 miles northeast of Damascus, borders Iraq and is one of Syria's richest provinces. It is the heart of Syria's oil-producing and grain-growing region and is home to a sizable share of the country's Kurdish minority.
The Syrian Observatory for Human Rights, an anti-Assad group based in Britain that reports on the conflict via a network of contacts inside Syria, said the rebel fighters were led by the Nusra Front, an Islamic militant group known for its combat skills but blacklisted by the United States for its suspected ties to Al Qaeda in Iraq.
The observatory said the Nusra Front had led a series of attacks on military and security posts in Hasaka, culminating in an assault on their bases in Shadadah. The observatory also said that rebels had stormed the administration and housing buildings of Syrian Oil Company workers at the al-Jbeysa fields, the largest in the province, and now controlled those facilities.
At least 100 members of the armed forces were killed in Shadadah, the observatory said, while 30 Nusra fighters were killed, including five from Kuwait and Iraq.
An anti-Assad Syrian activist in the region reached by phone, Omar Abu Layla, said casualties were high on both sides during three days of clashes, which included the Nusra Front's detonation of two car bombs outside the headquarters of the state security and military in Shadadah.
Mr. Abu Layla said the loss of Shadadah in particular was a setback for Mr. Assad's forces, which had sought to fortify the city against an insurgent advance. And the loss of the al-Jbeysa fields, he said, "is very important since they supply regime forces with oil needed to operate heavy equipment like tanks, for example."
With the fall of Shadadah, he said, "the regime could lose complete control of the province of Hasaka."
He also said the insurgents had seized a trove of ammunition and weapons, including antiaircraft guns, rocket launchers, machine guns, rocket-propelled grenades, two tanks, 400 Kalashnikov rifles and more than 500 cars, mostly sport utility vehicles.
Syria's official SANA news agency made no mention of Hasaka or the downed warplanes in its daily account of insurgent fighting, which, in keeping with government practice, described the rebels as terrorists. SANA said the military claimed a string of victories against terrorist cells in suburban Damascus, confiscation of Israeli-made weapons by terrorists in Homs and a deadly clash between two rival terrorist groups arguing over looted goods in Idlib Province.
Hania Mourtada reported from Beirut, and Rick Gladstone from New York. Hwaida Saad and Anne Barnard contributed reporting from Beirut, and Hala Droubi from Dubai, United Arab Emirates.
Correction: February 14, 2013, Thursday
This article has been revised to reflect the following correction: An earlier version of this article misspelled the names of the city and the major oil field facilities in Syria's northeast Hasaka province that insurgents claim to have captured. The city is Shadadah, not Shadadi, and the oil field facilities are al-Jbeysa, not al-Jabsa.
This article originally appeared in The New York Times.