Moscow has been Syria's key protector throughout the 17-month uprising that has evolved into a full-blown civil war, shielding Assad's regime from international sanctions and providing it with weapons despite an international outcry.
Russia's Foreign Ministry said the West "has done nothing" to urge the Syrian opposition to start a dialogue with the government.
"Instead, they are engaged in openly instigating it to continue their armed struggle," it said in a statement.
It claimed that the Western approach to the Syrian civil war is "hypocritical" and is not helping to resolve the conflict that has killed an estimated 19,000 people.
Russia has said that its opposition to sanctions against Syria is driven not by support for Assad himself, but by a respect for international law that forbids foreign military intervention in internal conflicts without U.N. Security Council authorization.
Russia has called for talks between the Syrian regime and its foes. It staunchly opposes any plans that would demand Assad's ouster, saying that only the Syrian people can decide the country's fate.
He said about 2.5 million people are in need of humanitarian assistance while the number of displaced people in Syria and the flow of refugees to neighboring countries is growing.
U.N. humanitarian chief Valerie Amos, who visited Syria last week, told a news conference at U.N. headquarters that U.N. agencies last month provided food for more than 820,000 people across Syria.
"But when you're talking about 2.5 million people affected, we need to do a lot more," she said.
Feltman and Amos appealed for additional funds to meet the humanitarian needs of Syrians still in the country and the refugees.
During her visit, Amos said she appealed to the Syrian government to allow international aid agencies to deliver food and humanitarian supplies to the needy.
But she said the government will only authorize aid from the Syrian Arab Red Crescent and local partners because it is worried that international aid will go to the rebels — despite her repeated assurances that humanitarian aid is neutral and impartial.
The deeply divided Security Council is scheduled to hold a ministerial-level meeting on Aug. 30 on the humanitarian situation in Syria.
Associated Press writer Edith M. Lederer contributed to this report from the United Nations.