Ben Emmerson, the UN special rapporteur on human rights and counter-terrorism, said the Pakistani government made clear to him that it does not consent to the strikes — a position that has been disputed by US officials.
The strikes have caused growing controversy because of the secrecy surrounding them and claims that they have caused significant civilian casualties — allegations denied by the United States.
According to a UN statement that Emmerson emailed to The Associated Press on Friday, the Pakistani government told him it has confirmed at least 400 civilian deaths by US drones on its territory. The statement was initially released on Thursday, following the investigator's three-day visit to Pakistan, which ended Wednesday. The visit was kept secret until Emmerson left.
Imtiaz Gul, an expert on Pakistani militancy who is helping Emmerson's team, said Friday that the organization he runs, the Center for Research and Security Studies, gave the UN investigator during his visit case studies on 25 strikes that allegedly killed around 200 civilians.
The UN investigation into civilian casualties from drone strikes and other targeted killings in Pakistan and several other countries was launched in January and is expected to deliver its conclusions in October.
The US rarely discusses the strikes in public because of their covert nature, but officials have said privately that they have caused very few civilian casualties.
A 2012 investigation by the AP into 10 of the deadliest recent drone strikes in Pakistan found that a significant majority of the casualties were militants, but civilians were also being killed.
Pakistani officials regularly criticize the attacks in public as a violation of the country's sovereignty, a popular position in a country where anti-American sentiment runs high.
But the reality has been more complicated in the past.
For many years, Pakistan allowed US drones to take off from bases within the country. Documents released by WikiLeaks in 2010 showed that senior Pakistani officials consented to the strikes in private to US diplomats, while at the same time condemning them in public.
Cooperation has certainly waned since then as the relationship between Pakistan and the US has deteriorated. In 2011, Pakistan kicked the US out of an air base used by American drones in the country's southwest, in retaliation for US airstrikes that killed 24 Pakistani soldiers.
But US officials insist privately that cooperation has not ended altogether, and key Pakistani military officers and civilian politicians continue to consent to the strikes.
However, Emmerson, the UN investigator, came away with a black and white view after his meetings with Pakistani officials.
"The position of the government of Pakistan is quite clear," said Emmerson. "It does not consent to the use of drones by the United States on its territory and it considers this to be a violation of Pakistan's sovereignty and territorial integrity."
The 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," he said.
Pakistan claimed the drone strikes were radicalizing a new generation of militants and said it was capable of fighting the war against Islamist extremism in the country by itself, said Emmerson.
A major reason why the US has stepped up drone attacks in Pakistan is because it has failed to convince the government to target Taliban militants using its territory to launch cross-border attacks against American troops in Afghanistan.
Emmerson met with a variety of Pakistani officials during his visit, as well as tribal leaders from the North Waziristan tribal area — the main target for US drones in the country — and locals who claimed they were injured by the attacks or had lost loved ones.
The tribal leaders said innocent tribesmen were often mistakenly targeted by drones because they were indistinguishable from Taliban militants, said Emmerson. Both groups wear the same traditional tribal clothing and normally carry a gun at all times, he said.
"It is time for the international community to heed the concerns of Pakistan, and give the next democratically elected government of Pakistan the space, support and assistance it needs to deliver a lasting peace on its own territory without forcible military interference by other states," said Emmerson.