The UN Syria special envoy said on Monday he was concerned by the slow progress of talks warning that without dialog beginning on a political transition it would be difficult to sustain the cessation of hostilities and aid deliveries.Speaking after talks with the head of the Syrian government delegation Bashar Ja'afari, Staffan de Mistura said that Ja'afari had told him it was too early to discuss political transition."He said it was premature at the moment to talk about it. Premature means imminent as far as we (UN) is concerned," de Mistura told reporters."Everyone more or less agrees, the cessation of hostilities is still holding and frankly largely. The same ... more or less for the movement on humanitarian aid. But neither of them can be sustained if we don't get progress on the political transition."