"There has been a very important offer of negotiation by Khatib of the National Coalition. It is important that that offer is responded to with serious negotiations by the Assad regime," British Foreign Secretary William Hague told reporters after talks in Beirut with Lebanese President Michel Sleiman.
"A political agreement on a transition is the only way forward to bring to an end this terrible and unacceptable loss of life," he said of the nearly two-year conflict which the UN says has claimed around 70,000 lives.
Hague also called on Assad to step down.
"It's time to go," he said, when asked what his message was for Assad.
"The people of Syria... have experienced enough suffering. Such destruction, such loss of life, such a threat to the stability of the whole region should not be endured because one person wishes to stay in power," he said.
National Coalition chief Khatib at the end of January offered to negotiate with regime officials who have no "blood on their hands."
Hague also announced a $17-million (12.9-million-euro) aid package for Syrian refugees in Lebanon, bringing Britain's total contribution to $30 million.
Speaking at the World Food Programme distribution centre in the Beirut suburb of Bourj Hammoud, Hague admitted that humanitarian aid was not enough.
"What we need above all is a solution to the crisis in Syria, and this is no substitute for that. We continue to work every day on trying to find that diplomatic and political solution.
"But in the absence of a solution, we have to do everything we can to help people in urgent need," Hague said.
Of the more than 800,000 Syrians who have fled to neighbouring countries, Lebanon has taken in the highest number -- nearly 300,000 people who live scattered across the country in the absence of refugee camps.
According to Doctors Without Borders, more than half of the Syrian refugees in Lebanon live in substandard structures with little to no protection from the elements and struggle to pay rent after losing their livelihoods.
Hague, who arrived in Beirut on Wednesday and also met Premier Najib Mikati, expressed British support for stability in Lebanon, which is sharply divided between supporters and opponents of the Assad regime.
He also said Britain would increase military assistance to the Lebanese military and would help train more than 2,000 troops in the coming year.