Secretary of State John Kerry and Foreign Minister Sergey V. Lavrov of Russia raised expectations in January at a joint news conference in Paris that a way would be found to open humanitarian aid corridors and possibly establish local cease-fires in Aleppo and other cities and towns.

But to the dismay of the United Nations and other humanitarian organizations, even those basic steps proved elusive…

A senior American official said the Syrian government had thwarted progress at the talks and refused to lift the blockade on the delivery of food and medicine to besieged areas, except for the delivery of aid to the Palestinian refugee camp at Yarmouk.