* No evidence of rebel use - PM Cameron's spokesman
* Kerry says Assad cannot be part of transition government
* Israel asks Russia not to sell Syria defence missiles
Britain said on Thursday it was "very likely" the Syrian government had used chemical weapons, and Turkey announced it was stepping up testing of people fleeing the Syrian civil war for traces.
U.S. Secretary of State John 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 United States and Russia this week. Syrian opposition leaders are sceptical of an initiative they fear might let President Bashar al-Assad hang on to power.
Kerry, in Rome, said however a transition government would have to have the "mutual consent of both sides, which clearly means that in our judgment President Assad will not be a component of that transitional government".
Syria's foreign ministry said Damascus was convinced by the "the firm Russian stance which is based on the U.N. principles of non-interference in internal affairs or the threat to use force against the safety of any state".
Israel has asked Russia not to sell Syria an advanced air defence system which would help Assad fend off foreign military intervention as he battles a more than two-year-old rebellion.
The S-300 missile is designed to shoot down planes and missiles at 125-mile (200-km) ranges. It would enhance Syria's current Russian-supplied defences, which failed to deter Israel from launching air strikes around Damascus last weekend.
"We have raised objections to this (sale) with the Russians, and the Americans have too," an Israeli official told Reuters.
Kerry said in Rome that Washington would prefer Russia not to sell weapons to Syria.
Israel said its air raids on Syria were intended to stop Damascus sending powerful Iranian missiles to Hezbollah fighters in Lebanon.
Hezbollah leader Hassan Nasrallah said in a speech on Thursday that Syria would respond to the raids by providing his group with sophisticated weapons, and Hezbollah would back any Syrian effort to recapture the Israeli-occupied Golan Heights.
Western states have been reluctant to consider military action against Assad, but U.S. President Barack 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.
French Foreign Minister Laurent Fabius said meanwhile the United Nations should declare Syria's Islamist militant al-Nusra Front a terrorist organisation, to differentiate it from other Syrian rebel groups.
France wants to bolster the opposition Syrian National Coalition, while pushing it to expand, unify and guarantee that a new government in Damascus would respect the rights of all communities, Fabius said in an interview with Le Monde daily.
Britain, which has suggested Syria's official opposition should be exempt from an EU arms embargo, said on Thursday there was a strong case for amending or lifting the ban when it expires on June 1.
A Foreign Office spokesman stressed Britain had not yet decided whether to supply arms to the opposition, but leaving the option open would help pressure Assad into joining talks.
"We need to find ways to increase pressure on the regime to accept a political solution and ensure we have flexibility to react to the deteriorating situation on the ground," he said.