HOMS, Syria—Syrian President Bashar al-Assad has personally stepped in to try to salvage the faltering United Nations humanitarian mission here.

Syrian allies Iran and Russia are pressing Mr. Assad to cooperate with U.N.-mediated peace talks in Geneva, a Western diplomat said. As part of the diplomacy, the allies also want the regime to build trust by easing suffering and allowing operations such as the one in Homs.

The governor of Homs earlier this week indicated that he was under pressure from Geneva to not impede the relief effort, which has brought food to starving civilians in a rebel-held enclave of Homs that has been besieged by government forces for more than 18 months. It has also evacuated nearly 1,500 people since it began on Friday.

But the mission has been beset by deadly violence, and concern is mounting over the fate of hundreds of men detained by the regime upon their evacuation because they were between the ages of 16-54 and presumed to be combatants.

One man evacuated on Wednesday conceded that he was scared about what would happen to him if he was detained.