Pre-reacted YBa₂Cu₃O_y has been treated under a pressure of 4 GPa in a belt apparatus and at high temperature (950 degrees C). The sample was characterized by X-ray diffraction, resistivity and magnetization (up to 5 T) measurements. The thermal treatment under high pressure preserves the mean oxygen content and restores fairly good junctions between the grains which have been partly crushed to fill the voids during the compression stage. This contrasts with previous results obtained after non-isostatic pressure treatments which led to semiconducting samples. The intra- and intergrain properties are discussed and compared with those observed on samples prepared under ordinary pressures.