UNITED NATIONS — Empowered by the Security Council to defy what United Nations officials have called an arbitrary aid blockade imposed by the warring parties in Syria, United Nations relief agencies for the first time sent a convoy of trucks carrying food, water purification tablets and other relief supplies across the Turkish border on Thursday into rebel-held areas of Syria.

The government in Damascus had repeatedly refused permission to United Nations agencies seeking to transport aid across Syria’s borders to rebel-held areas, effectively depriving civilians who live outside government-controlled territory of food and medicines.

The United Nations repeatedly pressed the Security Council to allow its trucks to enter without Syrian government consent, specifically through four border crossings that would enable its agencies to reach the largest number of civilians in need.

After extensive negotiations led by three Council members — Australia, Jordan and Luxembourg — the Council unanimously passed a measure on July 14 that authorized United Nations convoys to enter Syria through two crossings in Turkey, one in Iraq and one in Jordan, all beyond the Syrian government’s control.