London(CNN) Russia bears responsibility for Syrian President Bashar al-Assad's actions and his failure thus far to agree to a ceasefire in his country's 5-year-old conflict, U.S. Defense Secretary Ash Carter told CNN's Christiane Amanpour on Wednesday.
"It could use its influence to help put an end to this civil war," Carter told her in an interview in London. "They bear the responsibility of the consequences of things that they could avoid."
Carter noted Tuesday's attack in east Aleppo, allegedly involving barrels of chlorine gas dropped on a rebel-held neighborhood, as a sign that "things are definitely not heading in the right direction."
Medical groups and activists said that attack sent more than 100 people to the hospital.
Expressing further skepticism of Russia, Carter said that while "you've got to keep hoping, the experience suggests that we're not close to that point" where Moscow will change its actions.
On the relationship between two US allies fighting ISIS in Syria -- Turkey and the Syrian Kurds -- he acknowledged they "don't get along."
But the secretary said that they are key in the battle against ISIS and that the United States intends to "keep our commitments" to both of them.
"We work with both sides, and we try to manage the tension, which we understand," Carter said.
A key way to doing so, he said, is for each side to have operational awareness of the other -- "to know exactly what they're doing, and for us to establish the way with them, ways that they cannot interfere with one another in the pursuit of their separate objectives."
"So for example, we have agreed with them about where each party will be geographically in such a way that they can conduct their operations against ISIL (ISIS) and not run into each other, which might create a circumstance in which there could be a collision between the two of them, which we don't want to see."