ANKARA (Reuters) - Turkish authorities have arrested 12 people on suspicion of being members of a terrorist organization in raids across the country, a provincial governor said on Thursday.

The police raids were carried out in Turkey’s largest city Istanbul as well as in the southern provinces of Mersin, Adana and Hatay near the Syrian border, said Adana governor Huseyin Avni Cos.

Cos said unknown chemical materials were found during the raids and sent away for investigation. He denied media reports that a small amount of the nerve agent sarin had been uncovered.

Six of those arrested were later released, Cos said, while the other six were being kept for further questioning. He did not say which organization had been targeted in the raids.

Earlier, several Turkish newspapers had reported that 12 people from Syria’s al Qaeda-linked al-Nusra Front who allegedly had been planning an attack inside Turkey and were in possession of 2 kg (4.5 pounds) of sarin, had been detained in Adana.

The raids highlight the growing concern that Syria’s civil war is dragging in neighboring states.

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

Nusra is one of the most effective forces fighting President Bashar al-Assad and last month pledged allegiance to al Qaeda leader Ayman al-Zawahri. The U.S. State Department designated Nusra as a terrorist organization in December.

Experts have long said Nusra is receiving support from al Qaeda-linked militants in neighboring Iraq. The group claimed responsibility for deadly bombings in Damascus and Aleppo, and its fighters have joined other Syrian rebel brigades.

Assad’s forces and opposing rebels have accused each other of using chemical weapons, and both have denied it.

The United States has said it would view any use of chemical weapons in Syria as a “red line”, hinting that it could lead to some form of intervention, but wants proof before taking any action.