Arab Islamists and Kurdish militants, two of the most controversial armed groups in Syria's civil war, for the past week have been battling each other for control of key towns close to the Turkish border after the Kurds announced a plan to declare local autonomy.
The United States views both sides in the conflict, the Kurdish Democratic Union and the Nusra Front, as terrorist organizations, and the prospect of either prevailing is causing deep concern in Washington and Ankara, the Turkish capital.
Dozens have died in fighting and Nusra Front suicide bombings in what threatens to become a war within a war.
The Kurdish militia captured the border town of Ras al Ayn on Sunday, which had been under rebel control since last November. A day later, the United States said it was "very concerned" at reports that the militia was about to declare self-rule in northern Syria. "Such a declaration is highly provocative, as it will certainly exacerbate tensions between Arabs and Kurds," Jen Psaki, a State Department spokeswoman, said Monday.
Turkey, which embarked on a broad peace process in March with Kurdish militants at home, has warned that it won't tolerate an autonomous Kurdish province across the Syrian border. This week, the Turkish army warned that it had taken "all necessary precautions against threats across the border" and the deputy prime minister, Bulent Arinc, implied that the establishment of a de facto Kurdish entity in northern Syria might put Syria's territorial integrity in jeopardy.
"We have said that we are not going to accept any de facto entities that would be established in Syria by any ethnic groups or sects," he said.
The fear is that a Kurdish entity in Syria would revive demands for a Kurdish nation made up of predominantly Kurdish areas of Turkey, Syria and Iran in combination with the autonomous Kurdistan Regional Government in Iraq.
The other risk in the fighting is that if the Nusra Front prevails, it might be able, with the area's mineral resources, to set up a self-sustaining autonomous al Qaida state.
Gen. Salim Idriss, the head of the Syrian rebels' Supreme Military Council, which the United States has said should be the primary recipient of military aid, issued a warning about the risks of either the Kurdish Democratic Union, known by its initials as the PYD, or Nusra prevailing.
"The main goal of the PYD is to found their own state, 'Western Kurdistan,' " Idriss said Monday on Turkish television. "They are receiving support from the Syrian regime and Kurdish militants based in Iraq and Iran, and the PKK," a group that's been fighting the Turkish government for three decades seeking Kurdish autonomy.
Idriss said his group "won't ever accept or recognize such a state" and "will fight anyone who is making efforts to divide Syria." He said his group, whose loosely affiliated rebel units are referred to as the Free Syrian Army, was sending fighters to the region.
What precisely happened in Ras al Ayn is unclear. Idriss said the Kurdish Democratic Union had taken advantage of rebel forces being distracted by battles at al Malikiyah and over a military airport at Qamishli to take control of the town.
Turkish news media reported that Idriss traveled to Ankara to confer with Turkish officials. The Syrian Opposition Coalition, the political wing of the opposition, said Idriss then went to France to inquire why it hadn't yet shipped the sophisticated French-produced Milan anti-tank missiles that had been expected.
The Kurdish group that helped provoke the crisis is the Syrian branch of the PKK, whose name in English is the Kurdistan Workers' Party. The United States and the European Union list the PKK as a terrorist group.
Controversy surrounds the Kurdish Democratic Union, because although it says it opposes the government of Syrian President Bashar Assad, it's widely suspected that the group coordinated with the Syrian government when the regime withdrew its soldiers from large parts of the predominantly Kurdish area last year and allowed the Kurdish militia to fill the vacuum. The Kurdish force operates only a short distance from Syrian state security in Qamishli, the most important Kurdish city in Syria.
Kurds are about 10 percent of Syria's 23 million population, and Kurdish Democratic Union officials have been saying for some time that they intended to set up what they described as "an independent council to run Kurdish regions."
Salah Muslim, the head of the Kurdish militia, reaffirmed the plan Friday, which was to have been the day for the declaration. "This is not a call for a separation," he said in an interview with France 24 television. "It's just that for a year now we have been on our own in our own territories, and people have needs. They want some kind of administration to run their issues. They can't be left like that."
July 19 is the anniversary of the militia's takeover of the first Kurdish city in Syria, but it wasn't clear if the militia followed through on its plan to declare a local entity.
The Nusra Front, which has pledged allegiance to al Qaida leader Ayman al Zawahiri, has been the most militarily effective of Syria's anti-Assad rebel groups. This year it's seized much of Syria's oil and gas reserves, which are largely in the country's northeast.
Shortly after rebels seized control of Ras al Ayn from government forces last November, it became clear that Nusra made up the bulk of the fighters that had taken over. Within days, Nusra and Kurdish militias were battling.
By Roy Gutman
McClatchy Foreign Staff