Turkey claims evidence of Syrian chemical weapons use 10 May 2013 Last updated at 17:54 Turkish Prime Minister Recep Tayyip Erdogan has told US media he has evidence that Syria employed chemical weapons against opposition forces. He cited as proof the discovery of missile remains and added that Syrian patients brought to Turkey "were wounded by these chemical weapons." Mr Erdogan rejected the idea that the weapons could have been used by rebels. This contradicts an earlier claim by a UN expert who said there were "concrete suspicions" that rebels used nerve gas. Carla del Ponte, who serves on the Commission of Inquiry on Syria, said testimony from victims strongly suggested that opposition fighters had used sarin, an extremely potent chemical nerve agent - although there was "no incontrovertible proof." However, the commission later stressed that it had "not reached conclusive findings" as to their use by any parties. Red line 'crossed' Mr Erdogan said he did not think that the rebels had access to chemical weapons. "There is no way I can believe in this now," he said in an interview with American broadcaster NBC. First of all, how are they going to obtain this? And who will give this to them? But if it exists, we are against this. We are against whoever holds the weapons. He said Turkish intelligence had determined that the government of President Bashar al-Assad had used at least 200 chemical missiles. Chemical weapons in 20th century Chemical agents used as weapons was first introduced by Germany in WWI Mustard gas has been used in several wars including by British forces in Russian Civil War of 1919, Soviet forces in China in 1930s, Spanish and Italian troops in North Africa Sarin was invented by a German scientist in the 1930s in preparation for WWII Iraq's former leader Saddam Hussein used nerve agents such as sarin and mustard gas on Kurds in 1987-8 and on Iran 1980-8 Japanese militant sect Aum Shinrikyo used sarin nerve agent in a Tokyo subway in 1995 "We have the remainders of these missiles, there are pictures and then there are intelligence reports," he said. And there are patients who are brought to our hospitals who were wounded by these chemical weapons. He did not give details on the type of chemical weapons he believed Syria had used. Last month, the White House said intelligence agencies believed "with varying degrees of confidence" that Syria has used chemical weapons against rebels. President Barack Obama warned that chemical weapons use would be a "red line" for possible intervention, but said existent intelligence did not represent sufficient proof. The Turkish prime minister, however, said he believed that Syria had crossed the red line "a long time ago." Mr Erdogan, who is due to meet Mr Obama next week, called on the US to take stronger action. We want the United States to assume more responsibilities and take further steps. And what sort of steps they will take, we are going to talk about this. Limited but persuasive Meanwhile, British Prime Minister David Cameron has held talks in Moscow with Russian President Vladimir Putin, one of Syria's closest allies. Mr Cameron said the two countries had made "real progress" in discussions he described as "substantive, purposeful and frank." He said they had a "common interest" in stabilising Syria and preventing the growth of extremism. Mr Cameron also welcomed a recent agreement by Russia to convene an international conference to find a political solution to the crisis. The conference will try to persuade Mr Assad and the opposition to accept the establishment of a transitional government. In a separate development, a senior US official confirmed that the US ambassador to Syria briefly crossed into Syrian territory on Thursday, more than a year after the US embassy suspended operations in Damascus and pulled out its diplomats. The official told the BBC that Robert Ford was at the Syria-Turkey border to help deliver meals to Syrian opposition forces and discuss "the situation on the ground." The official added that the visit had no legal implications because Washington "has always continued to say that he remains the ambassador to the Syrian people, and that the regime is illegitimate." More than 70,000 people are believed to have died since the Syrian conflict erupted in March 2011.