Kofi Annan has ended his trip to Syria without a deal as regime forces mount a new assault on rebel strongholds in the north.
He is the highest-ranking international official to visit Syria to try to broker a peace agreement and said he had presented President Bashar Assad with concrete proposals "which will have a real impact on the ground".
"Once it's agreed, it will help launch the process and help end the crisis on the ground," Mr Annan told reporters at the end of his two-day visit.
Mr Annan, who also met with Syrian opposition leaders and businessmen in Damascus, said he was optimistic following two sets of talks with the president, but acknowledged that resolving the crisis would be tough.
"It's going to be difficult but we have to have hope," he said.
The former UN chief called for reforms that would create "a solid foundation for a democratic Syria," but added: "You have to start by stopping the killing and the misery and the abuse that is going on today and then give time for a political settlement."
The ongoing bloodshed cast a pall over the UN efforts to end the country's year-long conflict, with both the regime and the opposition refusing talks with the other.
In his discussions with President Assad, Mr Annan made several proposals to end the political crisis and start a political dialogue. He was rebuffed by the president, who rejected any immediate negotiations with the opposition, striking a further blow to already faltering international efforts for talks to end the conflict.
Mr Assad told Mr Annan that a political solution is impossible as long as "terrorist groups" threaten the country.
The opposition's political leadership has also rejected dialogue, saying talk is impossible after a crackdown that the UN estimates has killed more than 7,500 people. That makes it likely that the conflict will continue to edge toward civil war.