"Look, let's be frank, what we've done over the last 18 months hasn't been enough," Cameron said in remarks for release on Thursday. "The slaughter continues, the bloodshed is appalling, the bad effects it's having on the region, the radicalization, but also the humanitarian crisis that is engulfing Syria. So let's work together on really pushing what more we can do," he added.Western countries have been thwarted from taking strong action at the United Nations Security Council by Russia and China, who have vetoed tough draft resolutions against Syrian President Bashar Assad.It is unclear how tougher action could be taken against Assad given Russian and Chinese resistance, but analysts have suggested unilateral action might be an option.The prime minister visited a camp for Syrian refugees in Jordan on Wednesday, and a day earlier had suggested a safe exit from Syria could be arranged for Assad to end the violence, even if that meant he escaped a court trial.In August, the United States and Turkey indicated they were studying a range of possible measures, including a no-fly zone, and on Wednesday Turkey said it would request that NATO station Patriot missiles along its border with Syria.Also on Wednesday, Britain said it would begin contacts with Syrian opposition military figures, although British officials said that would not involve giving military guidance.The Syrian opposition estimates some 38,000 people have been killed since a March 2011 uprising against Assad's rule, the initially peaceful protests turning into armed rebellion in response to crackdowns by Assad's forces.