GENEVA (AP) — U.S. Secretary of State John Kerry on Monday said "several proposals" aimed at finding a way to restore at least a partial truce in Syria are being discussed, amid continuing attacks in the city of Aleppo.

Kerry met with Saudi Foreign Minister Adel al-Jubeir and with the U.N. envoy for Syria, Staffan de Mistura. He said progress was being made toward an understanding on how to reduce the violence in Aleppo but that more work was needed.

"There are several proposals that are now going back to key players to sign off," Kerry said after meeting de Mistura. We are hopeful but we are not there yet... we are going to work very hard in the next 24 hours, 48 hours to get there."

He did not say what the proposals were, adding that he would telephone Russia's Foreign Minister Sergei Lavrov later Monday and that de Mistura was headed to Moscow on Tuesday for talks.

He said the United States and Russia have agreed that there will be additional personnel stationed in Geneva around the clock to make sure there is more accountability and a better ability to enforce the cessation of hostilities on a day-to-day basis.

For Aleppo, the U.S. is considering drawing up with the Russians a detailed map that would lay out "safe zones." Civilians and members of moderate opposition groups covered by the truce could find shelter from persistent attacks by Assad's military, which claims to be targeting terrorists. One U.S. official said "hard lines" would delineate specific areas and neighborhoods. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

It was not immediately clear whether Russia would accept such a plan or if Moscow could persuade the Assad government to respect the prospective zones. Some U.S. officials are skeptical of the chances for success, but also note that it is worth a try to at least reduce the violence.

U.S. officials also say the safe zones will not be "no-fly zones" per se. However, details, such as the locations of those, have not yet been agreed to and these issues will be discussed by Kerry and Lavrov in a phone call later Monday, as well as by Lavrov and de Mistura in Moscow.

Earlier, al-Jubeir called the situation in Aleppo with continued airstrikes an "outrage" and a criminal violation of humanitarian law. He said that Syrian President Bashar Assad would be held accountable for the attacks and would be removed from power either through a political process or by force.

"There is only one side that is flying airplanes, and that is Bashar al-Assad and his allies, so they are responsible for the massacre of women, children, and the elderly," he said. "They are responsible for the murder of doctors and medical personnel, and this situation, any way you slice it, will not stand. The world is not going to allow them to get away with this."

Kerry's meetings in Geneva came as Syria's state news agency said the military has extended its cease-fire around Damascus and opposition strongholds in the eastern suburbs for another 48 hours. It said the cessation of hostilities that was declared Friday around the capital and the coastal Latakia region, following two weeks of escalating violence around the country.

But it excludes Aleppo, where more than 250 people have died in shelling and airstrikes in the northern city over the last nine days, according to the Britain-based Syrian Observatory for Human Rights.

Russia's Tass news agency, meanwhile, quoted Russian Lt. Gen. Sergei Kuralenko, head of the Russian coordination center in Syria, as saying that talks are continuing about a cease-fire for Aleppo.