* Turkey testing blood samples from Syrian patients
* Officials say patients had breathing difficulties
* Syrian government, opposition blame each other over attack
* Washington says use of chemical weapons a "red line"
Turkey is testing blood samples taken from Syrian casualties brought over the border from fighting in recent days to determine whether they were victims of a chemical weapons attack, local government and health officials said on Wednesday.
The samples were sent to Turkey's forensic medicine institute after several Syrians with breathing difficulties were brought to a Turkish hospital on Monday in the town of Reyhanli in Hatay province along the Syrian border.
"We are taking the necessary precautions as we have received unconfirmed information on the use of chemical weapons," Reyhanli Mayor Huseyin Sanverdi told Reuters.
"So far I have not received confirmation from medical institutions but there is a possibility that the weapons were used and we have to act with caution in case," he said.
Sanverdi said the hospital in Reyhanli had taken emergency measures on Monday following the claims but that those had now been lifted. He added that Monday's patients had been brought from Idlib province in northern Syria.
U.S. President Barack Obama on Tuesday said there was evidence that chemical weapons had been used during Syria's two year conflict, but that it was not yet known how the chemical weapons were used, when they were used and who used them.
Washington has long said it views the use of chemical weapons in Syria as a "red line", but wary of the false intelligence that was used to justify the 2003 war in Iraq, it has said it wants proof before taking action.
Britain last week confirmed it had "limited but persuasive" information showing chemical weapons use in Syria, including sarin, evidence that the Foreign Office now says is "physiological" - from the bodies of chemical attack victims.
A Foreign Office spokesman said it was likely that Syria, and not the rebels, would be behind any such attack, and Britain added that it was working with the United Nations to harden up evidence of whether chemical weapons had been used.
Fighting in Syria, now entering its third year, has intensified in the last month with government forces attempting to roll back rebel advances. Some 70,000 people have now been killed in the civil war.
Each side has blamed the other for what they both said was a chemical attack in the city of Saraqeb in Idlib on Monday.
A senior Reyhanli health official, who spoke on condition of anonymity, confirmed Sanverdi's statement, saying the hospital carried out "emergency plans from time to time".
One hospital employee, who also declined to be named, described how the hospital had been sealed off into the night on Monday, with specialised emergency medical teams moving in to take over after 13 patients from Idlib were brought in.
"We were given special apparel but it was the emergency team which took care of those patients. Doctors suspected sarin or mustard gas because the patients had breathing difficulties," the employee said.
Another hospital employee said staff were ordered to stay back while the team intervened.
"This cannot be without reason," the second employee said.
Wassim Taha, a Syrian doctor from the Union of Syrian Medical Relief Organisations which runs hospitals for the Syrian opposition, said the patients were washed at the border because doctors feared they had come into contact with a form of gas.
A second Syrian doctor, Ubada Alabrash, who helps treat Syrian patients at Reyhanli hospital, said they also suspected the patients had been victims of a chemical attack because those escorting them to the border had exhibited similar symptoms.
Alabrash said blood samples from the patients had been sent for tests but that they had not been given the results.
"I don't think the Turkish government would hide the results from us, but I understand they must be careful with it because NATO and other international bodies are also involved in this issue," he said.
"Now we are waiting for the blood test results from Ankara, we have asked to be informed. We can only say after the test results if chemical weapons were used or not."