DIYARBAKIR - A man and a 15-year-old boy were killed by stray bullets shot from Syria in a Turkish border town and Turkish troops returned fire, officials said on Wednesday, in the most serious spillover of violence into Turkey in weeks.

The Turkish military said it acted in accordance with its rules of engagement after bullets fired from the adjacent Syrian town of Ras al-Ain hit police headquarters in the southeastern Turkish town of Ceylanpinar and houses in the town's center.

The incident, which took place on Tuesday, underscores growing concern that Syria's more than two-year-old civil war is dragging in neighboring states.

Kurdish fighters have been battling Islamist anti-government rebels in Ras al-Ain since Tuesday. Ceylanpinar sits only metres across the frontier from Ras al-Ain.

Health officials said earlier that the boy was undergoing surgery after being hit in the head by a bullet, but later confirmed he had died of his wounds on Wednesday. Security sources said clashes were still ongoing.

The Turkish military said a local government official had been lightly wounded by one bullet but made no mention of any other casualties.

It gave no further details of any targets it had struck inside Syria.

Turkish troops have stepped up return fire into Syria in recent weeks due to what officials have said was heightened tension along the border and increased activity by smugglers, many of them armed.

The Syrian Observatory for Human Rights, a group opposed to Syrian President Bashar Assad, said Kurdish armed men had taken control of most of Ras al-Ain from Islamist rebel fighters from the al Qaeda-linked Nusra Front.

The clashes between Kurdish fighters, who generally support the creation of an autonomous region within Syria, and Islamist Arabs started on Tuesday after Nusra fighters attacked a Kurdish patrol and took a gunman hostage, the Observatory said.

Clashes between Kurds affiliated with the Democratic Union Party (PYD), a Syrian Kurdish party with links to Kurdish militants in Turkey, and anti-Assad Syrian and foreign fighters have erupted since Kurds began asserting control over parts of the northeast from late last year.

Ras al-Ain, also known by its Kurdish name Serekani, and Ceylanpinar were once a single town under the Ottoman Empire before they were split after World War One, and both have Arab and Kurdish communities.

In the worst example of the spillover of violence into Turkey, 52 people were killed when twin car bombs ripped through Reyhanli, another border town, on May 11. Turkey accused Syria of involvement in the attacks but Damascus has denied any role.

Turkey, which is sheltering around 500,000 Syrian refugees, has become one of Assad's most vocal critics and has scrambled war planes along the border as stray gunfire and shelling hit its soil. The civil war began with anti-government protests in March 2011.

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