ISLAMABAD - A top U.N. investigator has criticized Washington's drone campaign against Islamic militants in Pakistan as a violation of the South Asian nation's sovereignty.

That stance echoes Islamabad's public condemnation of the tactic but is not expected to end the airstrikes.

Ben Emmerson, U.N. special rapporteur on counterterrorism and human rights, issued a statement Friday saying the U.S. drone campaign "involves the use of force on the territory of another state without its consent, and is therefore a violation of Pakistan's sovereignty."

"Pakistan has also been quite clear that it considers the drone campaign to be counterproductive and to be radicalizing a whole new generation," Emmerson said, "thereby perpetuating the problem of terrorism in the region."

Emmerson's remarks came after he made a three-day visit to Pakistan last week, meeting with top Pakistani officials as well as tribal elders and victims of drone strikes.

Pakistan's Foreign Ministry issued a statement Friday saying the government had clearly conveyed to Emmerson "that drone strikes are counterproductive, against international law and a violation of its sovereignty and territorial integrity."

Washington's use of drone missile strikes in northwestern Pakistan's rugged tribal regions is one of the thorniest issues in its long, troubled relationship with Islamabad. The Obama administration has defended the tactic as a vital tool against al-Qaida and Taliban militant leaders hunkered down in the badlands along the Afghan border.

Although Pakistan has always publicly opposed drone strikes, many analysts believe the country's civilian and military leaders see value in the tactic and tacitly allow the strikes to occur. A 2008 diplomatic cable obtained and posted on the WikiLeaks website cited then-Prime Minister Yousuf Raza Gilani's acquiescence to the drone campaign.