Rebels and analysts critical of Mr. Assad’s government say he has a well-established pattern of agreeing to diplomatic initiatives to buy time, only to go on escalating the fighting.

For example, when Mr. Assad accepted Arab League monitors in the country in late 2011 and early 2012, he also intensified his crackdown on opponents, and shortly afterward he began the large-scale bombardments of rebel-held areas, like the Baba Amr neighborhood of Homs, that have since become daily occurrences…

Several opposition members said that shipments of arms and ammunition to the rebels had increased, though they refused to discuss the source. But the shipments were of light weapons and were distributed only to selected groups, and they were not seen as likely to make much difference on the ground. The persistent lament of the rebels — that arms are only dribbled out to them, to prevent either side in the civil war from winning — grew more pronounced.

“We won’t get advanced weapons,” said Lt. Amr Firzat, a spokesman for a rebel brigade fighting in Aleppo Province. “Because that would mean we would achieve victories.”