BEIRUT A surge of clashes in Syria's oil-producing northeast has killed dozens of rebels and Kurdish fighters in the past two days, activists said on Thursday, in fighting that highlights a struggle for territory and resources.
Fighters from Syria's ethnic Kurdish minority - roughly 10 percent of the 23-million-strong population - have carved out an increasingly autonomous region near the frontiers with Iraq and Turkey.
Syrian Kurdish militants, particularly the armed wing of the Democratic Union Party (PYD), have repeatedly clashed with opposition fighters led by al Qaeda-linked units in the region as government forces retreated over the past year.
The fighting has underlined the growing complexity of Syria's conflict which started with largely peaceful protests against President Bashar al-Assad and degenerated into a civil war that has killed more than 100,000 people.
Divisions in Syria along ethnic and sectarian lines - as well as the rise of radical Islamist units that have come to dominate the rebel movement - have made Western powers including the United States more hesitant to get directly involved in the 2-1/2-year-old uprising.
The Kurdish PYD's military wing blamed al Qaeda-linked groups for the latest violence, saying fighters from the Nusra Front and Islamic State in Iraq and the Levant (ISIL) attacked a string of Kurdish villages in Hasaka province.
Heavy artillery and tanks were used, it said.
Estimates of the numbers killed varied. The Britain-based Syrian Observatory for Human Rights said at least 35 rebels and 13 Kurdish fighters had died over the last two days.
Rebels accuse their Kurdish opponents of collaborating with Assad's forces to secure autonomy - although activists said disputes over resources and territory were a bigger factor in recent clashes.
Munzir Ehmed, a Kurdish activist in the city of Qamishli on the Turkish border, said the Islamists were attacking to try and take control of oilfields and a border crossing under Kurdish control.
"This is not a political issue. There are divisions even between these rebel units over oil," he said, adding that seven Kurdish fighters and four fighters and a commander from Islamist brigades died in clashes near Qamishli.
Ankara has been particularly wary of Syrian Kurdish activities near its borders, fearing that they could embolden home-grown militants of the Kurdistan Workers' Party (PKK) that is demanding more autonomy in Turkey.
Iran, Turkey and Iraq also have large populations of Kurds. Kurds have an autonomous regional government in Iraq, where tens of thousands of Syrian Kurds have fled to escape the war.