* Fragmented Arab rebels make common cause against Kurds
* Fighting centred on town of Atma, near Turkish border
* Arab rebels seek to prevent Kurdish drive for secession
By Khaled Yacoub Oweis
AMMAN, Sept 26 (Reuters) - Syrian rebels and al Qaeda-linked fighters clashed with Kurds in northern Syria on Thursday, activists said, in a battle for territory highlighting the country's descent towards sectarian and ethnic fiefdoms after two years of war.
The heavy fighting in the town of Atma on the border with Turkey's Hatay province followed outbreaks of internecine conflict by rival rebel forces elsewhere, which have undermined their military campaign to topple President Bashar al-Assad.
Even Assad's Sunni Muslim Arab opponents look increasingly divided after 13 rebel groups this week rejected the authority of the opposition coalition in exile and called for new Islamist leadership.
At least 15 fighters have been killed in two days of clashes around Atma, activists said.
The fighting pits Syrian Kurds, alarmed by what they see as Islamist encroachment in northern Syria, against Arab rebels who suspect the Kurds of seeking secession.
Faced with what they see as a shared Kurdish threat, moderate Free Syrian Army rebels fought in Atma alongside the Islamic State for Iraq and the Levant (ISIL), an al Qaeda affiliate controlled by foreign jihadists, only a week after the two factions fought each other in another border town.
Activists in Atma said Free Syrian Army units had brought heavy guns into the town and started blasting positions to push back Kurdish militiamen who had advanced to within sniper range in the surrounding hills.
They said Kurdish tank shells fired from distance were hitting Atma, while Arab rebel heavy artillery was hitting Jindaris.
At least nine ISIL fighters have been killed since the battle erupted on Wednesday in a region between Atma and the Kurdish town of Jindaris, part of the mostly Kurdish olive growing region of Ifrin.
The Syrian Observatory for Human Rights, which monitors the violence in Syria, said one of the dead was an ISIL local commander from the United Arab Emirates.
Six members of the PYD, a Syrian Kurdish force with close links to the Kurdistan Workers' Party (PKK), which has fought for autonomy in neighbouring Turkey, were killed, activists said. The Kurdish news service Welati said three Kurdish civilians had been killed.
Kurds comprise around 10 percent of Syria's 23 million population. They are concentrated in Ifrin and other areas of the northwest, in parts of Damascus and in the northeastern oil producing area of Qamishli, where there has also been intense fighting between Kurds and Arab rebel units.
"There is a big degree of fragmentation among the rebels and within the al Qaeda fighters themselves. The situation has become so chaotic that today's enemy is tomorrow's foe and vice versa," said activist Abdallah Teif.
Even moderate Kurds say they support military efforts to fend off a rebel attack on Jindaris.
"These extremists are worse than the Assad regime. There is no excuse to back them," said Bahzad Ibrahim, a senior member of the Kurdish National Council, a grouping of moderate Kurdish factions that does not include the PYD.
"If the Kurdish forces moved close to Atma is it because of battlefield requirements, not because they have territorial ambitions," he said.
The late president Hafez al-Assad backed PKK militants fighting against Turkish forces. But his son Bashar crushed a Syrian Kurdish uprising in 2004, as he began improving ties with Turkey, and cracked down on PKK militants based in Syria.
However, since the uprising erupted in 2011, Assad has released Kurdish militant prisoners and pulled his forces from remote Kurdish regions to focus his military campaign around Damascus and central Syria.