A Pentagon plan to aid Syrian rebels is emerging as far smaller than advocates hoped, ramping up slowly over an extended period while offering no quick support to moderate fighters, who are losing ground both to the Assad regime and to jihadists.

President Barack Obama promised in May to work with Congress to raise support for the moderates. But critics inside and outside the administration say the limited steps he is taking are too modest to make a difference on the battlefield, reflecting his own and the Pentagon's reluctance...