Syrian rebels with al-Qaida-linked fighters claimed nearly full control of an oil-rich northeast region Friday, the third big gain claimed by rebels this week. The reported takeover of the hotly contested Hasaka province -- a fertile, rich area 375 miles northeast of Damascus near Iraq -- came with critical assistance from the Sunni Islamist and jihadi militant al-Nusra Front after three days of bitter fighting, the Britain-based opposition Syrian Observatory for Human Rights said. Al-Nusra Front, also called Jabhat al-Nusra, is a 13-month-old extremist Islamic group with 6,000 to 10,000 fighters, known for its combat skills. Washington designated it a terrorist group in December for suspected ties to al-Qaida in Iraq, which is reputed to want to marginalize Shiite Muslims and establish a \"pure Islamic state.\" Assad\'s Alawite sect is an offshoot of Shiite Islam. Al-Nusra Front -- which claimed responsibility for 43 suicide attacks in Syria last year -- accounts for about 9 percent of the Free Syrian Army\'s total fighters, up from 3 percent in August 2012 and 1 percent at the beginning of last year, a non-governmental group tied to a more moderate FSA wing told the U.S. State Department in November. The group\'s Nov. 30 message, cited by The Washington Post, warned the State Department of the extremists\' rise. British Foreign Secretary William Hague told a British defense and security think tank Thursday Syria had become \"the No. 1 destination for jihadists anywhere in the world today.\" At least 100 members of the Syrian armed forces and 30 Nusra fighters, including five from Kuwait and Iraq, were killed in a pitched battle for the strategic Hasaka province town of Shadadah near a giant oil field, the observatory said. With Shadadah\'s fall, \"the regime could lose complete control of the province of Hasaka,\" anti-Assad activist Omar Abu Layla told The New York Times. The rebel conquest, if confirmed, would be the third in a string of victories this week. Rebels took control of Syria\'s largest hydropower dam Monday and captured a military air base with much of its fleet still intact Tuesday, the observatory said. They separately claimed to have shot down three Syrian air force warplanes Thursday -- two in Idlib province in Syria\'s northwest near Turkey and the other in the western-central Hama province, the opposition Local Coordination Committees of Syria said. Online videos monitored by United Press International corroborated the shootdown assertions.