The United States has renewed its allegations that Syria has used outlawed chemical weapons against rebels in the country's two-year-old conflict. U.S. Secretary of State John Kerry said Friday that there was "strong evidence" that President Bashar al-Assad's troops employed poison gas against rebel forces. Still, Kerry pushed the importance of talks to settle the crisis. "It's not an easy path but it is a path I think we, as a matter of conscience, are obligated to go down," Kerry said during a Google+ Hangout appearance. The White House said late last month that the United States had established with "varying degrees of confidence" that a sarin gas attack had occurred in Syria. Turkish Foreign Minister Ahmet Davutoglu said from Amman on Friday that Syrian casualties treated in his country showed signs of being victims of chemical weapons. "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 UN agencies," Davutoglu said. The foreign minister's comments added to speculations that U.S. President Barack Obama's "red line" on the use of chemical weapons may have been crossed. Wary of the false intelligence used to justify the 2003 war in Iraq, Obama has asked for more facts to back up intelligence assessments before taking any action in Syria. If evidence of usage of poison gas is confirmed, it could lead to deeper Western intervention against Assad and overshadow international efforts to bring the sides to peace talks. The Syrian government and the opposition have been trading accusations over the use of poison gas. U.S. government sources said blood and soil samples indicated the presence of sarin, although it was not yet clear whether local commanders or the Syrian administration had ordered the weapon to be deployed. Sarin, a lethal neurotoxin that can be delivered as a gas in artillery shells, was made illegal by the United Nations' 1993 Chemical Weapons Convention. Meanwhile, a spark of hope for a possible political solution to Syria's crisis appeared on the horizon as Washington and Moscow agreed Wednesday to push for the implementation of the Geneva communique, and to convene an international conference that would engage both the Syrian government and the opposition. The move was hailed by the Syrian government and the opposition as well as the UN and the Arab League. Lakhdar Brahimi, the international envoy to Syria, called the proposed conference "the first hopeful news concerning that unhappy country in a very long time." Analysts say the United States is likely to pressure the Syrian rebels to take part in the conference, though the opposition has rejected any negotiations with Assad until he steps down from office. That was a major shift for Washington, which has long been a backer of the Syrian opposition and even helped the rebels with "non-lethal aid." Many voices in Washington have urged military intervention in Syria and remain opposed to political solution talks. Russia, meanwhile, has been supportive of Assad and insists on dialogues. The participation of Washington and Moscow in the conference, which would be held by the end of May, would constitute a "real guarantee" for finding a political settlement to the crisis, analysts say. Also on Friday, Russian President Vladimir Putin and British Prime Minister David Cameron agreed to work toward a transitional government in Syria. "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 Cameron. France welcomed the talks between Moscow and Washington on a negotiated settlement in Syria, saying a political solution to the crisis is a priority. However, France insisted that Assad must stay out of power. Kerry took a similar stance Friday as he noted explicitly that a potential transitional government would not include Assad. "Because the terms of the agreement of the first Geneva conference are that the transitional government has to be chosen by mutual consent, and obviously, the opposition will never consent to Assad," Kerry said.