The Munich meeting comes amid increasing concern over Russian airstrikes in support of a Syrian ground offensive against Aleppo, which has bolstered President Bashar al-Assad's forces and which the U.S. and its allies fear spells death for efforts to reach a political solution to the bloody five-year civil war.
"Russia's activities in Aleppo and in the region right now are making it much more difficult to be able to come to the table and to be able to have a serious conversation," Kerry said Tuesday before leaving Wednesday morning for Munich.
On Tuesday, Russian Foreign Minister Sergey Lavrov said Moscow had submitted a "concrete plan" to end the fighting that Washington was reviewing.
U.S. officials said the Russian paper was similar to ideas that Kerry and Lavrov have been discussing over the past several weeks but acknowledged it would be difficult to see a cease-fire come out of Thursday's meeting.
"We don't have agreement on anything right now except that there needs to be a cease-fire," a senior State Department official said. "All of the other details, the date, who would be party to it, have to be worked out."
Talks about a cease-fire and humanitarian access have intensified over the past several weeks. The U.S. had hoped to have a ceasefire before the resumption of U.N.-led peace talks on February 25.
The talks began briefly at the end of last month but were suspended by U.N. Special Envoy for Syria Staffan di Mistura after only a few days, when the Russian offensive in Aleppo began, in order to stop the peace process from imploding.
"Through the ongoing onslaught on Aleppo, Russia seeks to tip the balance of power on the ground in favor of the Assad regime, with the aim of imposing a political solution favorable to Russia," said Khalid Saleh, a member of the main Syrian opposition National Coalition of Syrian Revolution and Opposition Forces.
With Russian and Syrian military operations cutting off supply routes of food, water and shelter, U.S. officials and Western and Arab diplomats said the immediate focus is securing humanitarian access to populations in besieged areas, like Aleppo and Madaya.
On Wednesday, several members of the U.N. Security Council pressed Russia to stop bombing around Aleppo to allow humanitarian access before Thursday.
"The modalities still need to be worked out, but we are working with the Russians to get some sort of accommodation," one senior State Department official said.
The International Committee of the Red Cross said Wednesday it estimates that around 50,000 people have been displaced by the increasing fighting in the northern parts of Aleppo province. In addition to needing food, water and shelter, the body warned of a shortage of fuel and electricity during extremely low winter temperatures.
The Pentagon warned Wednesday that the destruction of the city's two main hospitals from Russian airstrikes means "over 50,000 Syrians are now without any access to live-saving assistance," according to Steve Warren.
And the U.N. is warning that hundreds of thousands could be cut off from food if Syrian forces encircle the rebel-held parts of Aleppo.
A U.N. report earlier this week charged the Syrian regime with "inhumane actions" against Syrian civilians on a scale that "amounts to extermination" by denying access to humanitarian convoys.
On Tuesday, Kerry called on Russia to "join in the effort to bring about an immediate ceasefire and full humanitarian access.
Detailing the dire conditions, Kerry declared, "Russia needs to join with all of us in understanding that this cannot go on."
U.S. officials declined to get into the specifics of the ongoing negotiations out of fears of jeopardizing the outcome, but they said aid delivery could include airdrops of assistance by Russian and American aircraft. Land delivery has also been discussed, which diplomats said would need to be overseen by U.N. workers on the ground.
U.S. and other diplomats said the United Nations is expected to have a key role in determining where the aid is most badly needed and guaranteeing it is delivered.
"The Russians claim that they are cutting off weapons-supply corridors, but they are actually cutting off humanitarian corridors, so at the very least they need to put their money where their mouth is and open up the humanitarian corridors immediately to all of these besieged areas that the U.N. has identified," Brett McGurk, the U.S. envoy to the anti-ISIS coalition, told a Congressional hearing Wednesday.
"The situation is totally unacceptable," he continued. "It's causing a humanitarian catastrophe, it is strengthening the regime of Assad, and all that does is fuel extremists on both sides of the sectarian divide."
Kerry said last week that Russia had "offered constructive ideas" about how it could be implemented before the resumption of peace talks and that Iran also backs a cessation of the fighting.
U.S. officials and diplomats said the outcome of Thursday's meeting of the 17-nation International Syria Support Group, which includes Russia, Iran and Saudi Arabia, could be key in determining whether the fragile peace process can get back on track, or the U.S. and Arab-backed Syrian opposition collapses altogether.
"This meeting will tell a lot about the road ahead," Kerry said. "We are all very, very aware of how critical this moment is, and Russia needs to contribute in significant ways to sustaining the ability of the opposition and others to come to the table and create an atmosphere within which you can actually have a negotiation."