He spoke in Munich, Germany, where top diplomats from more than a dozen countries, including the United States and Russia, met to hammer out a deal. But given the situation in Syria, it's not clear what the deal will lead to on the ground.
"I'm pleased to say that as a result today in Munich, we believe we have made progress on both the humanitarian front and the cessation of hostilities front, and these two fronts, this progress, has the potential -- fully implemented, fully followed through on -- to be able to change the daily lives of the Syrian people," Kerry said.
"First, we have agreed to accelerate and expand the delivery of humanitarian aid beginning immediately," he told reporters.
"Second, we have agreed to implement a nationwide cessation of hostilities to begin in a target of one week's time. That's ambitious, but everybody is determined to move as rapidly as possible to try to achieve this."
Hope amid an uncertain outcome
But it was not immediately clear what effect this agreement would have in a country that has been racked by hatred, violence, death and destruction for almost five years. People who regard each other as terrorists and murderers -- as do Syrian President Bashar al-Assad
and his opponents -- neither reconcile easily nor lay down their arms lightly.
And terrorist groups such as ISIS and al Nusra Front -- both extremist Sunni forces -- are not even party to the new agreement, though they have caused much mayhem in the country.
But not all observers are totally without hope.
"I am a little bit more optimistic than most others are that this deal could have some success," said Doris Carrion, a research associate in the Middle East and North Africa Program at the Chatham House think tank in London.
The reason for Carrion's rather tempered optimism? There seems to be greater commitment in the international community to keep this war from spiraling out of control, she said.
The Russians have been advancing: "Freezing the fighting where it is now really works for them," Carrion said.
And when international actors such as the United States hear regional powers like Turkey and Saudi Arabia
raising the prospect of deploying ground troops in Syria, they want to do whatever they can to maintain stability.
None of that means that peace is around the corner in a conflict that has killed nearly half a million people, laid waste to much of the country and set several million people to flight.
"There's a lot of reason for skepticism," Carrion said. "But if it was ever going to happen over the five years of the war, it would happen now."
A call for help for civilians
Others have goals more modest than peace, such as the delivery of humanitarian aid to people who desperately need it.
And Turkish Foreign Minister Mevlut Cavusoglu said Friday via Twitter that "stopping the airstrikes" was what was important.
The agreement to cease hostilities is an "important step," Cavusoglu said. "What is important now is embracing this opportunity, stopping the airstrikes, ceasing targeting civilians and providing humanitarian access."
On Thursday, Russian Foreign Minister Sergey Lavrov in Munich said Russia would continue its airstrikes on ISIS
, al Nusra and other affiliated terrorist groups, as they are not subject to the truce, while at the at the meeting of the 17-nation International Syria Support Group.
CNN global affairs correspondent Elise Labott described the agreement to cease hostilities as both significant and tenuous. She took note of Kerry's use of the phrase "cessation of hostilities," rather than "ceasefire."
"This is an incremental, step-by-step process. The first building block here, the humanitarian aid," said CNN international diplomatic editor Nic Robertson.
"That cessation that comes begets, possibly, better, stronger talks -- ultimately ... maybe a ceasefire," he said.
At least 35,000 to 40,000 refugees have come into camps on the Syrian side of the Turkey-Syria border in the latest flood of people fleeing Russian bombardment, Turkish Deputy Prime Minister Yalcin Akdogan said Friday at a news conference in the Turkish border town of Kilis.
Turkey will continue its open door policy, but "not because someone told us to," Akdogan said. "No one has the right to give us a lesson in humanity," he said, because "Turkey has been left alone to struggle on its own."
More than 2.6 million refugees are living in 26 camps inside Turkey, he told reporters at the news conference Friday.
'Actions on the ground'
Any potential ceasefire would not apply to terrorist organizations operating in Syria. And Kerry
stressed that the longer the conflict persists there, the more extremists have to gain.
He also said that the real test of talks will be whether all the parties involved honor their commitments and implement them. Ending the five-year civil war
will require a plan for a political transition, he said.
The Syrian uprising began in March 2011. At least 250,000 people have died and 12 million have been displaced because of the conflict, according to the United Nations.
"What we have here are words on paper. What we need to see in the next few days are actions on the ground," Kerry told reporters.
His point was echoed by Lavrov, the Russian foreign minister, who spoke at the same news conference in Munich.
Lavrov said that a ceasefire would be difficult but characterized what was announced Friday as a "step forward."
"We have a common determination to reduce the suffering of the Syrian people," he said.