CEYLANPINAR, Turkey: Syrian warplanes tore along the Turkish frontier on Monday and bombed the rebel-held town of Ras al-Ain just metres (yards) inside the border, sending scores of civilians scrambling for safety into Turkey.
The Local Coordination Committees, a Syrian grassroots opposition group, said 16 people had died in the air strikes. The pro-opposition Syrian Observatory for Human Rights put the death toll at 12, including seven Islamist militant fighters.
Helicopters also strafed targets near Ras al-Ain, which fell to rebels on Thursday during an advance into Syria's mixed Arab and Kurdish northeast.
The offensive has caused some of the biggest refugee movements since the Syrian conflict began nearly 20 months ago.
Though Turkey is reluctant to be drawn into a regional conflict, the proximity of Monday's bombing raids marked a fresh test of its pledge to defend itself from any violation of its territory or any spillover of violence from Syria.
One of the jets struck within metres of the barbed-wire fence that divides Ras al-Ain from the Turkish settlement of Ceylanpinar, sending up plumes of black smoke.
From a vantage point in Turkey close to the border, the warplane appeared at one point to enter Turkish airspace.
At a clinic in Ceylanpinar, doctors tended a small child covered in blood. Anxious residents crowded outside a teahouse, watching the bombing and helicopter.
"I thought the Turkish government said it wouldn't allow these helicopters to come so close to the border," said one Turk, who declined to be named. "Look, they're coming inside our border."
Turkish Foreign Minister Ahmet Davutoglu said the jets did not violate Turkish air space but that Turkey had informed the U.N. Security Council and NATO about the latest strikes.
"The Assad regime is responsible for what took place. The secondary responsibility sits with the U.N. Security Council for their inaction," Davutoglu was quoted as saying by the state-run Anatolian news agency.
Some 9,000 Syrians fled the fighting in Ras al-Ain into Turkey in one 24-hour period last week, swelling to over 120,000 the number of registered refugees in Turkish camps, with winter setting in. Tens of thousands more are unregistered and living in Turkish homes.
Turkey is growing increasingly concerned about security along its border with Syria, in an area of the southeast where Ankara is also fighting an emboldened Kurdish insurgency.
Ankara says it has fired back in retaliation for stray gunfire and mortar rounds landing on Turkish soil, and is talking to its NATO allies about the possible deployment of Patriot surface-to-air missiles near the border.
Turkey says this would be a defensive step, but it could also be a prelude to enforcing a no-fly zone in Syria to limit the reach of President Bashar al-Assad's air power. Western powers have so far been reluctant to take such a step.
NATO Secretary General Anders Fogh Rasmussen said on Monday the military alliance would "do what it takes to protect and defend Turkey, our ally".
"We have all plans in place to make sure that we can protect and defend Turkey and hopefully that way also deter to that attacks on Turkey will not take place," he said in Prague.
Ras al-Ain, 600 km (375 miles) from Damascus, is part of Syria's northeastern oil-producing province of Hasaka, home to many of Syria's million-strong Kurdish minority.
Syrian Kurds have largely stayed away from the anti-Assad revolt and fear that the mostly Sunni Muslim Arab rebels will ignore their aspirations for autonomy in any post-Assad era.