President Barack Obama said Monday "gaps in trust" between the U.S. and Russia are keeping the two countries from reaching a military agreement.
Obama and Russian President Vladimir Putin met at the Group of 20 summit in an effort to reach a cease-fire deal in Syria, but ultimately, both walked away without an agreement.
Secretary of State John Kerry and his Russian counterpart had attempted a similar negotiation earlier, but that effort failed as well.
Russia and the U.S. are fighting ISIS in Syria, though a civil war in the country has complicated those efforts.
Russia supports Syrian President Bashar al-Assad and his regime, while the U.S. doesn't. Al-Assad's forces, and in the past Russian planes, have bombed U.S.-backed Syrian rebels under the guise of fighting ISIS.
In a press conference after his meeting with Putin, Obama said, "We have had some productive conversations about what a real cessation of hostilities would look like to allow us to both focus our energies on common enemies. ... But given the gaps of trust that exist, that’s a tough negotiation."
Calls for a truce have grown more urgent as Syrian national forces fight rebel troops in Aleppo. Almost 2 million people in the city are without food, water and medical care, but al-Assad has largely refused to let the U.N. deliver aid.