VIENTIANE, Laos — The Obama administration’s latest effort to broker a cease-fire in Syria’s civil war fell short on Monday, after a 90-minute meeting between President Obama and President Vladimir V. Putin of Russia failed to resolve problems between them.

“Given the gaps of trust that exist, that’s a tough negotiation, and we haven’t yet closed the gaps in a way where we think it would actually work,” Mr. Obama declared at a news conference at the end of a Group of 20 summit meeting in Hangzhou, China.

He did not describe the points of contention. Other officials have said they involve technical issues like how to staff checkpoints in combat areas. But the checkered history of Syrian cease-fires — the United States agreed to one with Russia in February, only to watch it unravel weeks later — has left the president deeply leery.

On a day of diplomacy and history, Mr. Obama later flew to Laos, becoming the first sitting American president to visit this country, where children still stumble over unexploded bombs dropped by the United States during the Vietnam War. Having failed to halt the guns in Syria, Mr. Obama will seek reconciliation with the victims of a war long over.