The U.S. government's covert program using unmanned drones to strike at terrorists inside Pakistan is unlikely to stop or be changed, despite new criticism from a U.N. human rights expert. U.S. officials insist the CIA program has been an effective tool to take out insurgents along the Afghanistan-Pakistan border, particularly those hidden beyond the reach of the military. The stepped-up use of drones over the past year has shown no signs of slowing down and was credited this week with the killing inside Pakistan of al-Qaida's third in command. Al-Qaida acknowledged the death of Mustafa al-Yazid. The Obama administration does not acknowledge the secret program, but one senior U.S. official defended its use Wednesday, saying a careful and rigorous targeting process is used to avoid civilian casualties. The official, who is familiar with the operation, spoke on condition of anonymity because the program is classified. The program, which officials say has killed hundreds of insurgents in dozens of strikes during the past year, has been condemned by critics who say it may constitute illegal assassinations and violate international law. They argue that intelligence officers conducting the strikes could be at risk of prosecution for murder in other countries.