The International Committee of the Red Cross said it was concerned about the civilians trapped by conflict as they head into their third winter of war.
Robert Mardini, the ICRC's head of operations for the Middle East, said in an interview broadcast Wednesday the situation was dire for many Syrians.
"The Syrian crisis will be remembered as the conflict where there has been, and there is still, a lack of basic respect for the medical mission," he said.
U.N. peace envoy Lakhdar Brahimi said earlier this week Kuwait is planning to host a donor conference in January to raise funds for Syrian refugees.
"The only solution to the humanitarian crisis is a political solution to the crisis," he said in a statement.
The ICRC said Syria's population lacks the basic systems to cope with the coming winter. Civilians trapped in either government-controlled or rebel-controlled areas, where humanitarian agencies don't have access, are the "most vulnerable of the vulnerable," Mardini said.