GENEVA — Declaring the official beginning of the first Syria peace talks in two years, the United Nations mediator said Monday that they must show results to the country’s suffering civilians and he called on the outside powers that are helping drive the conflict to take steps to reduce the violence.

The mediator, Staffan de Mistura, also said his most important objective now was to keep both sides talking — even if they are not talking directly to each other.

Mr. de Mistura, a veteran Swedish-Italian diplomat with a four-decade career at the United Nations — and its third mediator in the Syria war — considered the negotiations underway after he met for two hours with members of Syria’s opposition delegation at the organization’s Geneva headquarters. He had met with the Syrian government representatives here on Friday.

Speaking to reporters afterward, Mr. de Mistura emphasized that he was paying attention to the calls by opposition representatives for an end to airstrikes, relief from sieges, and a release of political prisoners.