Syrian rebels have launched a fierce offensive against President Bashar al-Assad’s troops in four northern provinces, aiming to reverse a string of defeats, an NGO and rebels said Monday.

UN chief Ban Ki-moon, meanwhile, said humanitarian access to Syrians remains “extremely challenging” a month after the world body demanded a lifting of sieges and bombardments of residential areas.

The rebel offensive follows several victories for the regime along the Lebanese border — in the Qalamun region, north of Damascus, and around renowned Crusader castle Krak des Chevaliers, west of the central city of Homs.