GENEVA (AP) — Russia's top diplomat said Friday he was considering "calling it a day" on talks with the United States to forge a cease-fire in Syria, expressing frustration with what he described as a five-hour wait for some sort of understanding between the two countries.

After a day of negotiations in Syria, designed to hash out an end to more than five years of warfare between Syria's Russian-backed government and U.S.-supported rebels, Russian Foreign Minister Sergey Lavrov blamed Washington for the impasse. Still, he held out for some sort of answer before the end of the American workday Friday.

"We are thinking of calling it a day and meeting next week," Lavrov said at a plush hotel in Geneva, where he met with Kerry on-and-off for several hours. In a clear attempt to pressure the U.S. for an answer, Lavrov joked that "it takes five hours for our friends to check with Washington" and offered sympathies to the large crowd of reporters gathered around him.

A senior administration official confirmed that the U.S. was locked in internal discussions.

Kerry was still involved in discussions with officials in Washington about the status of the proposals, said the U.S. official, who wasn't authorized to be quoted by name and demanded anonymity.

He didn't specify any of the sticking points, but the administration has been divided over the peace plan, which hinges on a U.S.-Russian military partnership to target the Islamic State and al-Qaida in Syria. Defense Secretary Ash Carter and National Intelligence Director James Clapper have both publicly expressed misgivings.

The new alliance of sorts would start if Russia halts offensives by Syrian President Bashar Assad's government and the U.S. can persuade the "moderate" rebels to break ranks with the Nusra Front, al-Qaida's local affiliate, and other extremist groups.

Neither side has succeeded in doing its part despite months of diplomacy. And the task may be getting even more difficult as fighting rages around the divided city of Aleppo, Syria's most populous and the new focus of a conflict that has killed as many as 500,000 people.

Assad's government appeared to tighten its siege of the former Syrian commercial hub on Thursday, following several gains over the weekend. Forty days of fighting in Aleppo has killed nearly 700 civilians, including 160 children, according to a Syrian human rights group. Volunteer first-responders said they pulled the bodies of nine people, including four children, from the rubble following air raids Friday on a rebel-held area. Reports suggested helicopters dropped crude barrel bombs over the area.

Even before Friday's negotiations began, senior U.S. officials sought to tamp down expectations of an imminent deal — even though other officials had said for days that Kerry wouldn't travel to Geneva for another round of diplomacy with Lavrov unless an agreement was clearly at hand.

Aleppo was to be a large part of the day's discussions, the officials said, along with the technical details of a cease-fire that define everything from how far back from demilitarized areas combatants would have to stay to the types of weapons they would need to withdraw from front lines. The officials, who briefed reporters traveling on Kerry's plane, were not authorized to discuss the developments publicly and spoke on condition of anonymity.

Since Aug. 26, Kerry and Lavrov have now met twice each in Geneva and in China on the sidelines of a global economic summit. They've held a flurry of phone calls in recent days. Both governments had said they were close to a package that would go beyond several previous truces between the Syrian government and armed opposition — all of which failed to hold. For Kerry, securing a sustainable peace in Syria has become his biggest objective as America's top diplomat since last summer's Iran nuclear deal.

In addition to those killed, Syria's conflict has chased millions of people from their homes, contributing to Europe's worst refugee crisis since World War II. Amid the chaos of fighting between Syria's government and rebels, the Islamic State group has emerged as a global terror threat.