Iran and Russia have never claimed ignorance of the criminality they facilitate.
Tens of thousands of people are being held in government custody, according to the U.N.
A prerequisite for peace is an end to barrel bombs, starvation sieges, mass terror and ubiquitous torture.
The Syrian conflict has killed more than 200,000 people and will enter its sixth year in March.
The message from Putin was relayed by the head of Russia's GRU military intelligence agency.
A convoy of 50 aid trucks will travel to the encircled Syrian town on Thursday.
The rebels want all sides in Syria's civil war to allow humanitarian access to those in need and cease attacks on civilians.
The aid said Damascus approved of U.N. resolutions passed last week endorsing an international road map for peace.
Over 100 political and armed opposition factions met to determine Syria's fate.
An Iranian news agency said yes on Wednesday, but authorities in Tehran and Moscow deny the meeting took place.
As American and French influence declines, the Assad-Iran-Hezbollah-Russia axis rises.