ISLAMABAD, Pakistan — The United Nations on Thursday stepped up pressure on Pakistan over the fate of hundreds of people who have disappeared into the illegal custody of the country’s powerful intelligence and law enforcement agencies over the last decade. A two-person delegation, led by a French law professor, Olivier de Frouville, spent 10 days meeting with government officials and about 100 people who said their relatives had been illegally abducted and, in some cases, tortured and killed. But in a sign of the delicacy of the subject, the leadership of the powerful Inter-Services Intelligence agency and the paramilitary Frontier Corps, which have been blamed for many of the disappearances, refused to meet the United Nations team. The current spate of disappearances started after 2001, when American officials pushed their Pakistani counterparts to abduct people suspected of being militants with Al Qaeda. But in recent years the phenomenon has been concentrated in the sprawling western province of Baluchistan, where a national insurgency has been under way since about 2006. Estimates of the scale of the problem vary enormously, from fewer than 100 missing people acknowledged by the provincial government of Baluchistan, to more than 14,000 claimed by some campaigning groups.