AMMAN (Reuters) - Syrian casualties treated in Turkey show signs of being victims of chemical weapons, the Turkish foreign minister said on Friday, adding to indications that President Barack Obama's "red line" on the use of such arms may have been crossed.
Wary of the false intelligence used to justify the 2003 war in Iraq, the United States says it wants proof that chemical weapons have been used before taking any action in Syria.
But if the evidence is confirmed it would increase the possibility of Western intervention against Syrian President Bashar al-Assad to stop the two-year civil war, even as the United States and Russia try to bring the sides to peace talks.
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 U.N. 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.
The U.S. president this week did not rule out action, military or otherwise, against Assad's government, but repeatedly stressed he would not be pressured prematurely into deeper intervention in Syria.
While Syria denies using chemical weapons, U.S. government sources said blood and soil samples indicate the use of the banned nerve agent sarin. It was not clear yet though whether local commanders or the government had ordered its use.
As the outside world deliberates, the conflict which has killed 70,000 people rages on unabated.
Twenty-five people were killed when the army shelled the central Syrian town of Halfayeh on Friday, opposition activists said. Video posted on the Internet showed panic-stricken residents attempting to cross a river to escape the bombardment.
What started as a series of peaceful protests against Assad spiraled into civil war pitting mainly Sunni Muslim insurgents against members of Assad's Alawite sect and other minorities.
Insurgents have seized many rural parts of Syria, most of the northern city of Aleppo and are pressing in on the capital Damascus, but Assad's forces have launched a fierce counter offensive in the past few weeks.
U.N. High Commissioner for Human Rights Navi Pillay said on Friday she was concerned by reports of a major military build-up by army and pro-Assad militia around the town of Qusair, near the Lebanese border.
Pillay said residents in Qusair feared a possible repeat of last week's events in the coastal village of Baida and the town of Banias in which activists said more than 100 people, including small children, were killed in a government advance.
"I am appalled at the apparent killing of women, children and men ... which seem to indicate a campaign targeting specific communities perceived to be supportive of the opposition," Pillay said.
Russia has supported Assad's government and supplied it with weapons, but agreed with the United States this week to help bring the sides together for an international peace conference.
Russian President Vladimir Putin and British Prime Minister David Cameron agreed on Friday to work towards a transitional government in Syria, despite acknowledging differences in their approach to the Middle Eastern country's civil war.
Cameron said international efforts envisaged "not just bringing the regime and opposition together at one negotiating table, but Britain, Russia, America and other countries helping shape a transitional government that all Syrians can trust to protect them."
Russia has been under pressure to cooperate more with Western powers at the U.N. Security Council to end the war.
"We have a common interest in the quickest end to the violence and the initiation of a peace process, and the preservation of Syria as a territorially whole sovereign state," Putin said after talks with the British prime minister.
Additional reporting by Oliver Holmes in Beirut, Tom Miles in Geneva and Denis Dyomkon in Sochi; Writing by Jon Hemming; Editing by Michael Roddy