World powers have been pushing for a halt in fighting that was meant to go into effect by Friday, but have struggled to agree on how it should be implemented.
Violence has intensified on the ground, with double car bomb blasts hitting the Al-Zahraa neighbourhood of the central city of Homs this morning.
The Britain-based Syrian Observatory for Human Rights said at least 46 people had been killed and dozens wounded in the explosions.
Homs city has regularly been targeted in bomb attacks, including last month when a double bombing claimed by the Islamic State group killed 22 people in Al-Zahraa.
The district's residents are mostly Alawites, the minority sect of Syria's ruling clan, including President Bashar al-Assad.
Most of those killed in the city have been civilians. In one of the deadliest attacks, 48 children and four adults died in blasts at a school in October 2014.
"We have reached a provisional agreement, in principle, on the terms of the cessation of hostilities that could begin in the coming days," Kerry said in Amman.
"It is not yet done and I anticipate that our presidents, President (Barack) Obama and President (Vladimir) Putin, may well speak somewhere in the next days or so in order to try to complete this task," he added.
World powers proposed the truce just over a week ago as part of a plan that also included expanded humanitarian access, in a bid to pave the way for the resumption of new peace talks.
Yesterday, a key opposition umbrella group, the High Negotiations Committee, said it would agree a temporary truce only if regime backers halted fire.
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