By Suleiman Al-Khalidi and Sylvia Westall
AMMAN/BEIRUT (Reuters) - Islamic State fighters advanced against rival insurgents in northern Syria on Sunday, capturing areas close to a border crossing with Turkey and threatening their supply route to Aleppo city, fighters and a group monitoring the war said.
Islamic State captured the town of Soran Azaz and two nearby villages after clashes with fighters from a northern rebel alliance, which includes both Western-backed rebels and Islamist fighters.
Islamic State will now be able to move along a road leading north to the Bab al-Salam crossing between the Syrian province of Aleppo and the Turkish province of Kilis, the Syrian Observatory for Human Rights said.
The town's loss is a blow to rebels grouped in the so-called Jabhat al-Shamiyya alliance (Levant Front), because it sits on an important supply route to bring weapons into eastern Aleppo, two fighters said.
"The main supply line between Turkey and Aleppo will be severely affected," Abu Bakr, an alliance field commander, said in a online message.
The Levant Front was created in Aleppo in an effort to forge unity among factions in Syria that have often fought each other as well as the Syrian army and hardline jihadist groups, undermining the revolt against President Bashar al-Assad.
Rebels said the Islamic State gains had upset plans for a wider offensive that was being prepared ahead of the Muslim holy month of Ramadan to seize government-controlled parts of Aleppo.
Residents in eastern Aleppo said convoys of rebel fighters were heading back to areas in the Soran countryside to repel Islamic State. Government forces hold the west of the city.
U.S-led coalition forces bombing Islamic State in Syria and Iraq carried out their latest raids near the city of Kobani close to Turkey and Syria's northeastern Hasaka province, but did not hit Aleppo and surrounding areas.
"(Islamic State) are heading to the Turkish border...If this happens I don't know who will be able to explain why the coalition is not bombing (them)," said Abo Abdo Salabman, a member of a rebel brigade which is not part of Levant Front but is fighting in the area.
"It's one thing that they don't bomb the regime because of some international circumstances they need to take into account. However, failure to bomb ISIS on that particular front is not going to go down well with the rebels," he said, warning that people were more likely to join hardline groups as a result.
Islamic State's next stop could be Syria's Azaz city, 10 km (6 miles) further north east and a gateway to the border crossing close by, the Observatory said.
"A small advance by Daesh (Islamic State) would get them to Azaz," said another rebel from the Nour al-Din al-Zenki brigade, which is in the Levant Front.
Azaz, flooded with thousands of refugees fleeing violence across northern Syria, has also been a major arms route and commercial thoroughfare for hundreds of trucks carrying Turkish goods to rebel-held areas in Aleppo and Idlib provinces.
Islamic State holds swathes of territory across Syria and Iraq, and has advanced rapidly in other parts of Syria in recent weeks, capturing the central city of Palmyra and the last border crossing between Syria and Iraq in the east.
The group is fighting both rival insurgents, the Syrian military and Kurdish forces in the four-year-old conflict.
Kurdish YPG forces have been battling Islamic State in Hasaka, a strategic province for all sides in the conflict due to its position next to Islamic State-held territory in Iraq.
Islamic State appeared to be losing land around Tal Abyad town, which lies north of its stronghold Raqqa city. Tal Abyad is one of the few remaining towns along the border with Turkey in Islamic State control, Kurdish and Arab tribal sources said.
Tal Abyad is Islamic State's main access point to Turkey from Raqqa.
(Additional reporting by Ahmed Tolba in Cairo; Editing by Angus MacSwan and Crispian Balmer)