Britain said on Thursday it believed it was "very likely" the Syrian government had used chemical weapons, and Turkey said it was testing victims of the Syrian civil war for traces of such substances.
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 skeptical 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 defense 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 defenses, which did not 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.
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, the 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 defense team, equipped with a specialist vehicle which can detect evidence of chemical, biological and nuclear substances, has been stationed at the Cilvegozu border gate near the town of Reyhanli, Anatolian said.
Turkey's Star newspaper, which is close to the government, reported on Thursday that the forensic institute that has been testing the blood samples had found traces of ricin, a highly toxic substance which can be used as a chemical warfare agent.
Turkish officials have declined to confirm whether the tests have been completed or to comment on any results.
French Foreign Minister Laurent Fabius said meanwhile the United Nations should declare Syria's Islamist militant al-Nusra Front a terrorist organization, 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.