However, he said more work was needed and there was no guarantee of success.
A cessation of hostilities has reduced fighting in Syria`s five-year civil war but has unravelled in recent days.
About 250 people have reportedly been killed in Aleppo in the past nine days.
On Monday, fresh government air strikes and artillery attacks on rebel-held districts and suburbs of Aleppo left at least three dead, according to activists.
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Mr Kerry, speaking alongside UN special envoy Staffan de Mistura after a morning of talks in Geneva, said the conflict was "in many ways out of control and deeply disturbing".
He said the nine-week-old cessation of hostilities had had "a profoundly positive effect" and saved many lives, but in recent weeks it had been "put to the test".
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