Even before the ceasefire was scheduled to take effect - sundown in Syria - the difficulties were underscored.
Syrian President Bashar al-Assad reiterated his determination to reconquer all of Syria from what he termed "terrorists," making it clear he has no plans to completely stop fighting to crush the five-year-old rebellion against his regime.
The pact spells out a process that intends - at least according to the Obama Administration - to culminate in Assad's departure.
"We as a nation . . . are delivering a message that the Syrian state is determined to recover all regions from the terrorists and restore security, infrastructure," Assad said after he attended prayers marking the Muslim Eid al-Adha holiday in Daraya, a Damascus suburb that was recently recaptured from the rebels after a four-year siege.
"We come today here to replace the fake freedom they tried to market at the beginning of the crisis . . . with real freedom," he added. "Not the freedom that begins with them and is sustained by dollars . . . and by some promises of positions."
He seemed to be referring to US backing of the rebels and opposition proposals to replace the Assad regime with a more representative government. In the announcement in Geneva, Kerry stressed that replacing Assad should be the ultimate goal of the deal reached with Russia, a key ally of the Syrian President.
The Syrian opposition has still not given a formal response to the ceasefire agreement, but rebel representatives say they have told US officials they plan to comply with the cessation of hostilities and expect to make an announcement in the coming hours.
Yasir Ibrahim al-Yusuf, a member of the political office of the rebel Noureddine al-Zinki movement, said the armed opposition has raised many concerns about the details of the deal with the Obama Administration, notably the absence of enforcement mechanisms or penalties for noncompliance by the Assad regime or the Russians.
A letter to the opposition delivered over the weekend by the US special envoy for Syria, Michael Ratney, spelled out details similar to those outlined by Kerry and Lavrov in Geneva. They include a cessation of hostilities, the delivery of humanitarian aid and the eventual launch of joint military operations by the US and Russia against terrorist groups.
The letter offered no new enforcement measures other than the reporting mechanism established by ceasefire agreement earlier this year, which collapsed within weeks amid escalating government airstrikes. The armed opposition is nonetheless committed to complying "because it is incredibly important that aid reaches people and that there is a decrease in the numbers of people dying," Yusuf said. "Also, we are hoping this is the beginning of a political solution to conflict."
THE KEY POINTS
1 An initial 48-hour ceasefire if successful, can be renewed for another 48 hours at a time.
2 Both sides will allow humanitarian access to all of the besieged and the hard-to-reach areas.
3 The Syrian regime is to refrain from flying combat missions in agreed opposition areas.
4 Jabhat Fatah al-Sham is excluded from the ceasefire deal.
5 If the truce holds for one week, the US and Russia would work together on strikes against jihadists such as Isis and Jabhat Fatah al-Sham.
The US has also sought to reassure the rebels that Russia is committed to the deal, he added.
"The one assurance we have is that the Russians are very invested because they want to extricate themselves from this conflict as quickly as possible," he said. "This is the one reason we are agreeing to the ceasefire. It seems everyone very much wants to make it work."
Syria's Government, meanwhile, has already indirectly said that it accepts the deal. Lavrov told reporters in Geneva that Assad has given his assent. The official government news agency SANA added that "sources underscored that the Syrian Government had been informed of the agreement and agreed to it," which appeared to imply acceptance.
Pro-government news organisations reported, however, that the Government would not accept all of the ceasefire's terms, including a requirement that pro-Assad forces retreat from a key road into Aleppo that was seized from the rebels nearly two months ago.
According to the timetable laid out by Kerry and Lavrov, if the ceasefire holds for seven consecutive days, and humanitarian aid flows unimpeded to besieged areas, then Moscow and Washington will start working out plans to conduct joint military operations.
One of the groups that could be increasingly targeted is Jabhat Fatah al-Sham, or Front for the Conquest of Syria, the former al-Qaeda affiliate previously known as Jabhat al-Nusra. The US letter to the rebels urges them to disengage from the Front, warning of "dire consequences" if they do not.
A surge of violence over the weekend in which at least 90 people died compounded scepticism that the ceasefire would result in much more than a temporary lull - and underlined why it is so important to stop the fighting.
The vast majority of the victims - at least 85 people - were killed by a wave of suspected government airstrikes against the rebel-held cities of Idlib and Aleppo, according to doctors in the two cities. On Monday, five more people died in another strike in Aleppo, two of them children, and warplanes returned again early today, doctors and residents said.
