The United States and Russia will meet in Geneva, Switzerland, in the coming days, Kerry said, to make sure moderate opposition forces are not attacked during the truce and to discuss continued strikes against ISIS.
Kerry said "a significant benefit" of this plan was that it might "speed up the destruction" of ISIS.
The United States has alternative plans in place in case efforts to create a transitional government in Syria do not work, which Kerry said would become apparent in a month or two.
"There are certainly Plan B options being considered," he said. Syrian President Bashar al-Assad "is going to have to make some real decisions about the formation of a transitional governance process that's real."
The truce was announced Monday in a joint statement
by Russia and the United States. The "cessation of hostilities" was the result of talks between the two countries as co-chairs of the International Syria Support Group.
The countries proposed that the truce begin at midnight Friday in Damascus (5 p.m. ET Friday).
The deadline for any group in Syria to indicate it is participating in the cessation of hostilities is noon Friday in Damascus. The agreement does not include ISIS, al-Nusra Front or other terrorist groups.
The main opposition group in Syria, the National Coalition of Syrian Revolution and Opposition Forces, issued a statement expressing "preliminary approval to reach a temporary truce."
The opposition group also called for guarantees that Russia, Iran and sectarian militias would stop the fighting.
The United States and Russia said they plan to share information on the groups pledging to put down their arms. Doing so is meant to ensure that U.S., Russian and Syrian forces do not attack groups abiding by the truce.
"The Russian Federation and the United States will establish a communication hotline and, if necessary and appropriate, a working group to exchange relevant information after the cessation of hostilities has gone into effect," their statement said.
"Russia will conduct the necessary work with Damascus and the legitimate Syrian leadership. We expect that the United States will do the same with regard to their allies and the groups they support," Russian President Vladimir Putin said in an address published on the Kremlin website.
'This is a moment of promise'
Kerry called the draft document promising but said in a statement
that its success requires compliance by all parties involved.
"If implemented and adhered to, this cessation will not only lead to a decline in violence, but also continue to expand the delivery of urgently needed humanitarian supplies to besieged areas and support a political transition to a government that is responsive to the desires of the Syrian people," he said.
"This is a moment of promise, but the fulfillment of that promise depends on actions."
Are there loopholes?
Despite the moment of promise, British officials said they were concerned that Syrian Kurds were now working with the Syrian regime and Russia.
"What we have seen over the last weeks is very disturbing evidence of coordination between Syrian Kurdish forces, the Syrian regime and the Russian air force, which are making us distinctly uneasy about the Kurds' role in all of this," British Foreign Secretary Philip Hammond said Tuesday in Parliament.
The terms of the agreement do not call for a total surrender of arms.
Warring parties are asked to commit "to proportionate use of force (i.e., no greater than required to address an immediate threat) if and when responding in self-defense."
Asked at a
whether this part of the agreement could be misused to continue attacks, U.S. State Department spokesman Mark Toner acknowledged that setting benchmarks and monitoring the cessation of hostilities would not be easy.
"There's not going to be any force on the ground or monitors on the ground to look at this, other than where we get information from in terms of intelligence, but also through (nongovernmental organizations) reporting on the ground and journalists, frankly, who give feedback as to who's being hit by whom," Toner said.
"I think the work in front of the task force over the next several days is to work out kind of standard operating procedures and working methods for the task force to implement the ceasefire," he said.
Skepticism over truce
Monday was not the first time a ceasefire date has been agreed upon in the Syrian crisis.
On February 12, Kerry announced
that world powers meeting in Munich, Germany, had agreed on a target date for a cessation of hostilities in Syria a week later.
That date came and went, and the fighting and deaths continued.
The group Physicians for Human Rights expressed skepticism that the latest truce would stop the violence on the ground in Syria.
"What Syrians need is a cessation of attacks on civilians, and there is no evidence that this agreement effectively addresses the bombing of civilian facilities such as hospitals, which have been repeatedly targeted by Syrian and Russian forces under the pretext of attacking terrorist groups," Widney Brown, director of programs for the doctors' group, said in a statement.
Almost 200 people were killed over the weekend
in multiple attacks in Homs and on the outskirts of Damascus. ISIS claimed responsibility for those attacks.
More than 250,000 people have been killed, more than 1 million injured and more than half of Syrians displaced since the country's civil war began in 2011, according to the United Nations.