DIYARBAKIR, Turkey — Turkey scrambled fighter jets along its border with Syria on Monday after Syrian government forces bombed rebel positions in the frontier town of Ras al-Ain and stray shells flew into Turkish territory, Turkish security sources said.

Rebels overran Ras al-Ain almost a month ago in fighting which has triggered some of the biggest refugee movements of Syria's 20-month civil war, and has tested Turkey's resolve to defend itself against any spillover of violence.

The security sources said Turkish F-16 jets were scrambled from their base in the southeastern city of Diyarbakir after the air raids on the Free Syrian Army's headquarters in Ras al-Ain.

Shells landed in the Turkish town of Ceylanpinar, which abuts Ras al-Ain, triggering panic, the sources said. It was not immediately clear whether the shells were fired by forces loyal to President Bashar al-Assad or by the rebels.

Turkey, a major backer of Syria's opposition, has asked NATO for Patriot surface-to-air missiles to be positioned near its 900-km (560-mile) border with Syria as a defensive measure.

Ankara is concerned about its neighbor's chemical weapons, the growing refugee crisis on its border, and what it says is Syrian support for Kurdish militants on its soil.