By Louis Charbonneau UNITED NATIONS (Reuters) - A U.N. report on how Syria's neediest civilians are often not accessible to humanitarian relief workers makes it clear that the government of President Bashar al-Assad shoulders most of the blame, a U.S. official said on Tuesday. A month after the 15-member U.N. Security Council achieved rare consensus to approve a resolution demanding rapid, safe and unhindered aid access in Syria, including across borders, U.N. chief Ban Ki-moon said in a new report that the situation "remains extremely challenging. His report, which was delivered to council members on Sunday but has not been officially released, criticized both the government and rebels for hindering access to civilians caught in the crossfire of the three-year civil war. But in Washington's view, Ban's report was especially damning for the government.