Russia's foreign minister sounded a pessimistic note Saturday on chances of securing a "cessation of hostilities" in Syria within a week as planned, putting the chances at less than 50 percent.

Appearing at a security conference in Munich a day after diplomats secured a deal to work toward the temporary truce and to speed deliveries of humanitarian aid to besieged Syrians, Sergey Lavrov indicated that Russia remains deeply suspicious of U.S. intentions.

He said everyday military cooperation between the U.S. and Russia in particular is "the key tool" to ensuring the delivery of humanitarian supplies and an end to hostilities in Syria. But Lavrov complained coordination hasn't gone beyond an agreement to avoid in-air incidents.

"If we are moving closer to practical goals of (a) truce, then without cooperation between the military nothing will work out," Lavrov said.

Pressed by conference moderator Wolfgang Ischinger to say how confident he is that a "cessation of hostilities" will be implemented within a week, Lavrov replied: "49" out of 100. British Foreign Secretary Philip Hammond, sitting alongside Lavrov, said the Russian's comments made the chances sound more like "somewhere close to zero."

Lavrov said comments by U.S. officials raised the impression that the aim is for the Russian military operation in Syria to be stopped "although we are fighting the very same ... organizations which the U.N. Security Council has designated as terrorist organizations."

Britain's Hammond said the chance of a truce "depends on what Russia wants."

Syrian government forces, aided by a Russian bombing campaign, are trying to encircle rebels in Syria's largest city of Aleppo and cut off their supply route to Turkey.

"Unless Russia over the next days is going to stop or at least significantly scale back that bombing, the moderate armed opposition will not join in this process," Hammond said. "They cannot be expected to join in this process."