No deal on Syria as US-Russia talks end.

Talks between the US and Russia have ended without an agreement on ending violence in Syria, a US official said on Monday.

Meeting on the sidelines of the Group of 20 summit here, US Secretary of State John Kerry and his Russian counterpart Sergey Lavrov were working to negotiate a plan that would have boosted military cooperation between the two nations in an effort to better target terrorists and prevent civilian deaths.

But negotiators failed to work though differences, and the talks have ended for now.

"There are still issues to resolve," the US official said. Neither side indicated what the points of contention were, or when they expected talks to resume.

On Sunday, cautious optimism prevailed that a deal could be struck between Washington and Moscow, long at odds over policy in Syria. Kerry and Lavrov had been working "around the clock" to come to an agreement, US President Barack Obama told reporters.

The campaign has spurred a humanitarian crisis and caused millions of Syrians to flee for Europe.

The US hopes to align with Russia to identify terrorist targets, including ISIS and the Nusra Front, a group formerly tied to al-Qaeda. Officials hope a ceasefire will help advance talks on a political transition that would lead to the resignation of Syrian leader Bashar al-Assad.

Obama and Russian President Vladimir Putin were expected to interact Monday on the sidelines of the G20 summit, though a formal meeting was not on the books.

Any deal to end widespread suffering in Syria was likely to be met with doubt after past settlements, including a ceasefire agreed to in February, failed -- a fact Obama acknowledged on

"Given the failure of previous cessations of hostilities to hold, we approach it with some skepticism," Obama said.

But he insisted any work toward easing the deeply troubling humanitarian crisis was valuable.

"It is worth trying," he said. "To the extent that there are children and women and innocent civilians who can get food and medical supplies and, you know, get some relief from the constant terror of bombings, that's worth the effort."