US Secretary of State John Kerry said there was 'strong evidence' that Syria had used chemical weapons during the prolonged conflict in the country, even as Turkey announced it was stepping up testing of people fleeing the Syrian civil war for traces.

Event Britain had said on Thursday that it was "very likely" the Syrian government had used chemical weapons.

US President Barack Obama has called time and again the use of or failure to secure chemical weapons by the Syrian government a "red line" or "game changer" in his handling of the conflict in the Middle East country.

But Obama has also asked for more facts to back up the intelligence assessment before taking further steps, while senior officials said in the past days the administration was considering providing weapons to the Syrian rebels, as demanded by some lawmakers.

The Syrian government and the armed rebels have been trading accusations over the use of chemical weapons in the country's deadly conflict.

Last December, the Syrian government warned that the rebels could use chemical weapons in their fight against President Bashar al-Assad's forces, while insisting that the government would never unleash such arms on its own people.

Besides talking about the use of chemical weapons, Kerry again called for political transition in Syria in order to stop the ongoing bloodshed.

Kerry expressed gratitude to Russia for its willingness to try to arrange a "Geneva two" conference to negotiate an end to the conflict, in a sign of a thawing of the long diplomatic chill between Washington and Moscow, Syria's strongest ally.

Damascus and the head of the Arab League welcomed the apparent rapprochement between the US and Russia this week. Syrian opposition leaders are skeptical of an initiative they fear might let President Bashar al-Assad hang on to power.

Western states have been reluctant to consider military action against Assad, but Obama has said the use of chemical weapons would cross a "red line" and trigger a strong response.

Evidence of such use is so far fragmentary and disputed.

Asked about reports that rebel forces had used the banned nerve agent sarin, a spokesman for British Prime Minister David Cameron said: "Our assessment is that chemical weapons use in Syria is very likely to have been initiated by the regime. We have no evidence to date of opposition use."

Turkey has sent a team of eight experts to the border with Syria to test wounded victims of the country's civil war for traces of chemical and biological weapons, state-run Anatolian news agency said.

Turkey started testing blood samples last week from Syrian casualties brought over the border for treatment to determine whether they were victims of a chemical weapons attack.

The civil defence team, equipped with a specialist vehicle which can detect chemical, biological and nuclear traces, has been stationed at the Cilvegozu border gate near the town of Reyhanli, Anatolian said.

Some Turkish newspapers said ricin, a highly toxic substance, had been detected.

The latest samples were taken from some 12 people from the Syrian province of Idlib who arrived in Turkey with breathing difficulties.

Foreign Minister Ahmet Davutoglu said it was too soon to draw conclusions.

"Examinations are continuing. When the final result is out, whatever that is, we will share this with the public and inform the relevant international institutions," he told a news conference in Ankara.

He said Ankara had been carrying out such tests for a while but would now examine every patient who arrived from Syria.-Reuters