Syrian warplanes bombed two rebel bases near the Turkish border on Monday sending hundreds of people fleeing across the frontier.
The attacks on the Free Syrian Army positions (FSA) in Atima and nearby Bab al-Hawa came a day before NATO and Ankara were due to start assessing where to station surface-to-air missiles on the Turkish side of the 900-km (560-mile) boundary.
Turkey, a big supporter of rebels fighting to oust Syria's President Bashar al-Assad, has repeatedly scrambled jets along the border. It has also responded in kind when shells from the conflict have landed inside Turkey, underlining fears Syria's 20-month-old revolt could spread to destabilise the region.
Ankara has rejected Syrian complaints that the NATO Patriots were "provocative" and stressed they would be used only to defend Turkish territory, not to create a no-fly zone inside Syria that rebels have long demanded as a way to neutralise Assad's massive air power.
Syrian planes dropped around six bombs on a rebel base near the Bab al-Hawa border crossing, said activists.
"There were lots of people injured ... I saw many wounded people on the border before I was brought here," FSA member Mahmoud Ahmad told Reuters after arriving in Turkey for treatment.
The attack also flattened tents being set up nearby for displaced people inside Syria by a Turkish charity, but no one was inside them at the time, Turkey's state-run Anadolu agency reported. Reuters television footage showed tattered shelters and a bomb crater near a line of tanks.
Two Syrian jets fired five rockets at an FSA base in Atima, around 2km (1 mile) from the border, said opposition activist Ahmed, who lives nearby. "Three have hit farm areas and another two hit buildings near the base."
Rebels fired anti-aircraft guns at the jets but they were flying too high to be hit, activists said. "I think the reason for the raid may have something to do with increased weapons movements (from Turkey)," Ahmed said.
Several hundred Syrians fled into Turkey after the Atima raid and were being taken care of by the Turkish army.
The Turkish Anatolian news agency said an anti-aircraft shell fired during clashes in another Syrian border town, Harem, hit the roof of a house in the Turkish district of Reyhanli but caused no casualties.
Syrian rebels have been tightening their hold on farmland and urban centres to the east and northeast of Damascus, and have seized a string of military bases in the past 10 days.
A joint Turkish-NATO team will start work on Tuesday assessing where to put Patriot missiles, how many will be needed and the number of foreign troops to be sent to operate them.
Turkey is reluctant to be drawn into the fighting, but the proximity of Syrian bombing raids to its border is straining its nerves. It is worried about its neighbour's chemical weapons, the refugee crisis on its border, and what it says is Syrian support for Kurdish militants on its own soil.
Activists say more than 40,000 people have been killed in Syria's civil war, which started with peaceful demonstrations for reform but grew into demands for the overthrow of 42 years of dynastic rule by Assad and his late father, Hafez al-Assad.
UN Secretary General Ban Ki-moon, speaking to the Austrian paper Der Standard, urged Assad to consider a political settlement with the opposition.
"The military option is not sustainable. The conflict should be resolved via a political process. (Assad) should realise that he has gone too far, too deep, and how can he continue this way? He should listen to what his people would like," he said.
Attacks by mainly Sunni Muslim rebels against Assad's forces have become increasingly effective and deadly. The president, from Syria's Alawite minority which is linked to Shi'ite Islam, has responded with devastating artillery and air bombardment.
Hundreds of thousands of Syrians have fled their country and more than 2 million more have been displaced. The opposition said last week $60 billion would be needed for reconstruction.
With winter coming, the suffering will grow as displaced families seek food, medicine and shelter. The UN chief said only 40 percent of needed humanitarian aid has been made available.Euphrates dam captured
The military installations rebels have captured in the last 10 days include a major facility in the northern province of Aleppo and several bases around the capital Damascus.
On Monday activists said rebels took control of the Tishreen dam on the Euphrates river, east of the city of Aleppo. Internet video footage showed gunmen inside what appeared to be the control room, undamaged following the rebel capture.
Other footage showed rebels opening up ammunition boxes, including one marked RPG (rocket-propelled grenades), which they said were seized from Assad's forces holding the dam.
On Sunday rebels said they had captured a helicopter base east of Damascus, their latest gain in a battle that is drawing nearer to Assad's seat of power in the capital.
The Marj al-Sultan base, 15 km (10 miles) from Damascus, is the second military facility on the outskirts of the city reported to fall to Assad's opponents this month. Activists said rebels destroyed two helicopters and taken 15 prisoners.
"We are coming for you Bashar!" a rebel shouted in an internet video of what activists said was Marj al-Sultan. Restrictions on non-state media meant it could not be verified.
The rebels have been tightening their hold on farmland and urban centres to the east and northeast of Damascus while a major battle has been under way for a week in the suburb of Daraya near the main highway south.
"We are seeing the starting signs of a rebel siege of Damascus," opposition campaigner Fawaz Tello said from Berlin. "Marj al-Sultan is very near to the Damascus Airport road and to the airport itself. The rebels appear to be heading toward cutting this as well as the main northern artery to Aleppo."
Assad's core forces, drawn mainly from his Alawite sect, are entrenched in the capital.
© Thomson Reuters 2012