On the surface, it sounds like long-awaited good news: The United States, Russia, and other world powers have agreed to begin a “cessation of hostilities’’ in Syria, to allow humanitarian aid to reach starving civilians under siege.

But the truce — slated to start in one week’s time — comes as the Syrian army and Russian planes rapidly advance on Aleppo, an opposition stronghold since 2012. By the time the agreement takes hold — if it takes hold at all — Syrian government troops may have already encircled tens of thousands of civilians in rebel-held parts of the city.

It’s apparent that Russians have only agreed to halt the war because this humanitarian pause in the fighting leaves them and their ally, Syrian President Bashar al-Assad, in control of valuable territory, giving them an advantage in future peace talks.

There’s another reason to be skeptical of this deal: According to some reports, Russia will play a key role in delivering food and medicine to besieged towns. This is worrisome because Russia has been complicit in the Assad regime’s use of food as a weapon of war. The international community must be vigilant and ensure that food is distributed to all civilians, not only those in places that have been loyal to Assad.

It’s no wonder that members of the exhausted Syrian Free Army express cynicism and even despair about the impending cease-fire.

“No one believes it,’’ Mohammed al-Sheikh, a spokesman for the Free Syrian Army told the Guardian newspaper. “Talking about a cease-fire has become a routine.’’

It’s hard to think of anyone who should be more aware of the downsides of this deal than Secretary of State John Kerry, who watched Russia sign a cease-fire in eastern Ukraine a year ago, only to break it within days.

Kerry should do all he can to ensure that all parties live up to this deal. But he should also prepare a “Plan B,’’ in the likely event that Russia and Syria don’t keep their promise to cease hostilities and allow the delivery of aid to civilians in rebel-held towns.

As part of this plan, the Obama administration should seriously consider creating a “safe zone’’ in northern Syria, by the Turkish border, to protect civilians from Assad and the Islamic State. To be sure, creating such a zone would be difficult and dangerous. Keeping out jihadist groups, who would doubtlessly try to target the zone, and sheltering potentially millions of people are just two of the myriad of challenges the United States and its allies would face. That’s the reason the White House has repeatedly rejected the idea.

But the cost of action must be weighed against the escalating price tag of inaction. As the United States faces increasing pressure from European and Middle Eastern allies to stem the tide of refugees, the case for a humanitarian zone becomes more compelling.

And although the Russian intervention has made the establishment of such a zone far trickier, it should not be viewed as impossible.

Two of America’s most seasoned diplomats — Nicholas Burns, a former US undersecretary of state for political affairs, and James Jeffrey, former US ambassador to Iraq and Turkey — argued recently in The Washington Post that Russia should be invited to participate in the coalition that secures the safe zone, but that the United States and its partners should establish one anyway, in the event that Russia refuses.

Perhaps the biggest reason to put a safe zone plan on the table is that it would show both Assad and the Syrian opposition that the United States is finally willing to take decisive action to end the Syrian civil war. If well-executed, it would bolster America’s dwindling leverage in the region. Diplomacy in the absence of a credible military option is just talk. With more than 250,000 dead and 12 million homeless, Syria desperately needs more than words.