Syrian casualties treated in Turkey show signs of being victims of chemical weapons, the Turkish Foreign Minister said on Friday, May 10, adding to indications that President Barack Obama's "red line" on the use of such arms may have been crossed, Reuters reported.

Turkey confirmed last week it was testing blood samples from Syrian casualties brought over the border to determine whether they had suffered a chemical weapons attack.

"We have been making tests and we have some indications regarding chemical weapons being used, but in order to make sure and verify we are continuing these tests and will be sharing these tests with UN agencies," Foreign Minister Ahmet Davutoglu said in the Jordanian capital Amman on Friday.

"We know the Syrian regime has stocks... And everybody knows the Syrian regime has this capacity," Davutoglu said. "Of course this has been one of our major concerns because chemical weapons are a threat against humanity and a crime."

Obama said in August he viewed the use of chemical weapons in Syria as a "red line". Turkish Prime Minister Tayyip Erdogan said that line had been crossed a "long time ago".

"There are patients who are brought to our hospitals who were wounded by these chemical weapons," he told NBC television. "You can see who is affected by chemical missiles by their burns."

Erdogan is due to meet Obama in Washington on May 16.