The U.N. envoy for Syria is expressing concern about a new trend: President Bashar Assad's government allows humanitarian aid into besieged cities and towns, and those same areas get shelled either before or after the convoys arrive.

Staffan de Mistura made the comments on Syria's five-year war by videoconference Tuesday from Geneva to the U.N. General Assembly plenary in New York.

De Mistura also detailed recent U.N. achievements in delivering aid to hundreds of thousands of Syrians and expressed international hopes to revive stalled Syrian peace talks in coming weeks.

He said a truce between Assad's forces and rebel fighters brokered by the U.S. and Russia is "heavily challenged" in places like Idlib, Latakia and Aleppo, but overall fighting remains less than before it was reached in February.