Turkish Prime Minister Recep Tayyip Erdogan has told an American television channel he believes Syria has used chemical weapons, crossing a "red line" set out by US President Barack Obama.
"It is clear the regime has used chemical weapons and missiles," Erdogan told NBC News on Thursday, without providing details on when or where they were used.
"It has been passed (a) long time ago," Erdogan said of Obama's "red line," calling for stronger US action against Syrian President Bashar al-Assad.
"We want the United States to assume more responsibilities and take further steps. And what sort of steps they will take, we are going to talk about this," said Erdogan, who will meet Obama on May 16.
Erdogan said Syrian patients showing signs of chemical attacks had been brought to hospitals across the border in Turkey, and that "remainders of missiles" that he believes were used in such attacks have been found.
Turkey's foreign minister, Ahmet Davutoglu, meanwhile told reporters that authorities were conducting blood tests on wounded Syrian refugees to assess whether their injuries had been caused by chemical weapons.
He said more than a dozen test results "required further study and should be taken seriously" but did not elaborate further. "The studies continue. We will publish the results once they are concluded," he added.
Western nations have raised concerns about the use of chemical weapons in the escalating conflict, now more than two years old, between Assad's regime and rebels fighting to oust him.
Top UN rights investigator Carla del Ponte said Sunday that according to testimony, rebels may have been using the deadly nerve agent sarin.
Both Erdogan and Davutoglu said there was no evidence that Syrian rebels had used chemical weapons.
Ankara has sided with rebels fighting to topple Assad's regime, taken in around 400,000 refugees as well as army defectors and repeatedly called on the international community to act on the unfolding crisis.