Ben Emmerson, the UN special rapporteur on human rights and counter-terrorism, said the Pakistani government told him that it does not consent to the strikes – a position that has been disputed by US officials.
President Barack Obama has stepped up covert Central Intelligence Agency drone strikes targeting al-Qaeda and Taleban militants in Pakistan’s tribal region along the Afghan border since he took office in 2009.
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 US.
According to a UN statement from Mr Emmerson, the Pakistani government told him it has confirmed at least 400 civilian deaths by US drones on its territory.
Imtiaz Gul, an expert on Pakistani militancy who is helping Mr Emmerson’s team, said that the organisation he runs, the Centre for Research and Security Studies, gave the UN investigator during his visit to Pakistan this week case studies on 25 strikes that have allegedly killed some 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 claimed privately that they have caused very few civilian casualties.
Pakistani officials regularly criticise the attacks in public as a violation of the country’s sovereignty, a popular position in a country where anti-American sentiment runs high.
However, the reality has been more complicated. 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.
Co-operation has certainly waned since then as the relationship between Pakistan and the US has deteriorated. In 2011, Pakistan kicked the US out of an airbase used by American drones in the country’s south-west, in retaliation for US airstrikes that killed 24 Pakistani soldiers.
However, US officials insist privately that co-operation has not ended altogether, and key Pakistani military officers and civilian politicians continue to consent to the strikes.
However, Mr Emmerson 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 Mr 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 radicalising a generation of militants and said it was capable of fighting the war against Islamist extremism in the country by itself, added Mr Emmerson.