* Turkish troops return fire into Syria for second day
* Capture will alarm Ankara, fearful of Kurdish power grab
* Ankara calls on U.N. to act after border violence
A Syrian Kurdish party with links to Kurdish militants in Turkey has seized control of a Syrian border town after days of clashes with Islamist fighters, the Turkish military said.
The capture of Ras al-Ain on Syria's northeastern border with Turkey by the Democratic Union Party (PYD) will heighten Ankara's fears that the emergence of an autonomous Kurdish region in Syria could embolden homegrown militants of the Kurdistan Workers' Party (PKK), which is fighting for autonomy in Turkey.
Turkey's foreign minister voiced concern at the spillover of violence from the war in its southern neighbour and called again on the United Nations Security Council, which has yet to come to a consensus over Syria, to act.
In a statement late on Wednesday, the military said Ras al-Ain had fallen under the control of the PYD, which it described as a "separatist terrorist organisation". Fighting in the town had now stopped.
Turkish troops had shot at PYD fighters in Syria in accordance with its rules of engagement after two rocket propelled grenades fired from Syria struck a border post on the Turkish side of the frontier.
The return fire was the second time in as many days the military has answered in kind after several stray bullets from Syria struck the police headquarters and several homes in the adjacent Turkish town of Ceylanpinar on Tuesday.
Two Turkish citizens, including a 15-year-old boy, were killed by stray bullets in what was the most serious spillover of violence into Turkey from Syria in weeks.
The military said it had now strengthened security along that part of the border with armoured vehicles.
The clashes between Kurdish fighters, who generally support the creation of an autonomous region within Syria, and Islamist Arab fighters from the al Qaeda-linked Nusra Front started on Tuesday after Nusra fighters attacked a Kurdish patrol, according to an anti-government Syrian activist group.
Clashes between Kurds affiliated with the PYD, and Syrian and foreign fighters opposed to Syrian President Bashar al-Assad, have erupted since Kurds began asserting control over parts of the northeast from late last year.
Turkey, with its own large Kurdish minority, has been watching closely, concerned a Kurdish power grab to the south could strengthen PKK militants in Turkey with whom they have embarked on a peace process.
Developments in Syria could threaten that process, which is already under pressure amid an increase in militant activity in Turkey seen as fuelled by frustration at Ankara's perceived lack of progress on pushing through related reforms.
Turkey's main pro-Kurdish party, the BDP, on Wednesday criticised Ankara over its Syria policy and said it worked contrary to peace efforts with Turkey's Kurds.
Speaking in Ankara on Wednesday, Foreign Minister Ahmet Davutoglu expressed concern over the events along the border.
"This illustrates a striking picture of how much the crisis in Syria can affect us and our citizens," state-run broadcaster TRT quoted Davutoglu as saying.
"Once again we call upon the international community...if the U.N. Security Council is to do the job it is required to do, then the moment is now," he said.
Turkey, which has emerged as one of Assad's most vocal critics and biggest backers of the rebels fighting to overthrow him, has hit out previously at the U.N. Security Council for failing to adopt a united stance on Syria.
Prime Minister Tayyip Erdogan was expected to meet President Abdullah Gul and the military's Chief of General Staff Necdet Ozel on Thursday and the latest developments in Syria were likely to feature high on the agenda.
Turkey, which has the second-largest army in NATO, is reluctant to act unilaterally in Syria although it has scrambled war planes along the border as gunfire and shelling hit its soil. Turkey hosts around 500,000 Syrian refugees.