Hangzhou (China): 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, CNN reported. But negotiators failed to work though differences, and the talks have ended for now. "There are still issues to resolve," CNN quoted the US official as saying. Neither side indicated what the points of contention were, or when they expected talks to resume. 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. Russian forces have aligned with Syrian President Bashar al-Assad to target US-backed opposition fighters that Moscow and Damascus claim are terrorists, CNN added. The campaign has spurred a humanitarian crisis and caused millions of Syrians to flee for Europe.