US and Russia are joined in Lausanne by UN and Middle East powers on different sides of the five-year conflict.
Fresh diplomatic talks to end the Syrian conflict are under way in Switzerland, the first since the United States halted bilateral negotiations with Russia on a ceasefire approved earlier this month.
With violence still raging in Aleppo and reports of new attacks on hospitals in the northern city, John Kerry, the US secretary of state, met Sergey Lavrov, Russia's foreign minister, and top diplomats from the United Nations and regional powers in Lausanne on Saturday.
Ahead of the talks, Vitaly Churkin, Russia's UN ambassador, said that a key aim of the Lausanne meeting is to get countries that support "moderate" opposition groups to use their influence to work for a new ceasefire.
Kerry and Lavrov are joined in Lausanne by Staffan de Mistura, the UN Syria envoy, along with the top diplomats of Turkey, Saudi Arabia and Qatar - all backers of Syrian opposition forces.
Iran, a key supporter of the Syrian government, has also sent its foreign minister, Mohammad Javad Zarif, to Lausanne to take part in the talks.