HOMS, Syria—An attempt to implement the second phase of a United Nations plan to evacuate civilians and take desperately needed aid into a besieged rebel-held area in this central Syrian city was marred by violence on Saturday, with one U.N. official describing what happened as "a day in hell."

One day after succeeding in evacuating 83 people trapped in sections of central Homs for more than 18 months, a convoy comprising U.N. and Syrian Arab Red Crescent vehicles was attacked with mortars and gunfire. The attack killed at least five people on the rebel side, wounded one Syrian aid worker and forced a U.N. team including its top representative in the country to remain trapped in the rebel-held sector for hours.

Opposition activists and one member of the aid team that went into the rebel-held area blamed pro-regime forces for the attack, while Homs Governor Talal al Barazi and several security and military officials blamed the rebels.

The aid trucks had attempted to take food rations, flour, medicine for chronic diseases and hygiene kits into the besieged parts of Homs, which encompass sections of the downtown area and what's known as the Old Quarter.

The plan was also to evacuate more civilians out of the estimated 2,500 believed still to be in these rebel-held areas. But only some of the aid ultimately went in, no civilians were able to leave and the entire mission was overshadowed by the ordeal of the U.N. team and aid workers.