The al Qaeda-linked Nusra Front attacked Western-backed opposition fighters in northern Syria on Friday, rebel groups and an organization monitoring the war said, escalating tension between rival insurgents near the Turkish border.The Syrian Observatory for Human Rights, the monitoring group, also reported that warplanes believed to be part of a U.S.-led alliance had bombed Nusra Front positions in the area near the town of Azaz, north of Aleppo city.The fighting, which the Observatory said had killed at least 13 people on both sides, points to one of the major complications facing U.S. and Turkish plans to jointly sweep ISIS terrorists from northern Syria.The Nusra Front, one of the most powerful insurgent groups in northern Syria, has a track record of crushing rebel groups that have received support from Western states, including the Hazzm movement that collapsed earlier this year.Opposition sources say that one of the rebel groups targeted in Nusra's overnight attack, Division 30, has been a participant in the newly launched U.S.-led program to train and equip insurgents to fight ISIS.Reports have claimed that the al-Qaeda linked Al-Nusra Front had detained 18 U.S. trained Syrian rebels, including Syrian Turkmen Colonel Nedim Hassan, -who is allegedly the leader of the U.S train-and-equip program-, and field commander Farhan Jasim near the Syrian city of Aleppo on July 29.Pentagon said on Wednesday that the Al-Nusra abductees are not members of U.S. train and equip program."While we will not disclose the names of specific groups involved with the Syria Train and Equip program I can confirm that there have been no New Syrian Force personnel captured or detained." Pentagon spokeswoman or Cmdr. Elissa Smith told Daily Sabah.One Turkish senior official speaking to Daily Sabah on Thursday also confirmed that there was no detention or abduction of US-trained Syrian rebel forces. "Those detained by Al-Nusra are not part of train and equip program", the official said.Division 30 said Nusra Front fighters attacked its headquarters at 4.30 am in the area near Azaz. Five members of Division 30 were killed as they held off the attackers, it said.The group has accused the Nusra Front of abducting its leader and several other members earlier this week.Syrian opposition sources say members of Division 30 have been trained under the U.S.-led train and equip program launched in May. The Pentagon has however cast doubt on the report, saying that no members of the "New Syrian Force" had been captured or detained.Another rebel group in the area said it had also clashed with Nusra Front in Azaz.Jaish al-Thuwar, a rebel alliance formed earlier this year said in a statement posted on Facebook on Friday four of its members and at least eight attackers had been killed.The Nusra Front, which Washington has designated a terrorist organization, last year routed the Syria Revolutionaries Front led by Jamal Maarouf, viewed as one of the most powerful insurgent leaders until his defeat.It was also instrumental in the demise of the U.S.-backed Hazzm Movement, which collapsed earlier this year after clashing with the Nusra Front in the northwest.Washington and Ankara this week announced their intention to provide air cover for Syrian rebels and jointly sweep ISIS fighters from a strip of land along the border, with U.S. warplanes using bases in Turkey for strikes.