Nor is a cessation the same as a full ceasefire, which requires verification mechanisms.
But he argued it was unreasonable to demand al-Nusra forces leave before a full ceasefire.
Mr Kerry cautioned that, if implemented, the deal would only achieve a "pause" in fighting and that more work would be needed to reach a full ceasefire.
Is Iran prepared to insure that President Assad and his forces engage in a full ceasefire, stop dropping barrel bombs, and stop the campaign against rebel groups?
The agreement stopped short of a full ceasefire because it was not signed by the main parties themselves – the Syrian government and the factions fighting against it.
Saudi Arabia, one of the chief backers of the Syrian opposition, had said it would increase the flow and quantity of weaponry to moderate fighters in Aleppo if Russia refused to agree to a full ceasefire.
One salient line from Hamas leader Khaled Meshaal's remarks earlier, flagged by Reuters and others: Meshaal said he was prepared to accept a "humanitarian truce" – although he would not agree to a full ceasefire until the terms had been negotiated.
The six-hour pause in Israeli air strikes began at 9am on Tuesday as part of a proposal unveiled by Egypt on Monday intended to lead to a full ceasefire and talks on a long-term agreement.
