Pakistani cricketer-turned-politician Imran Khan said he was pulled off a flight to New York by US agents on Friday and interrogated about his views on drone strikes.
Khan has been a vociferous opponent of killings by the unmanned US aircraft in Pakistan and has promised to instruct the Pakistani air force to shoot them down if he wins next year's elections.
"I was taken off from plane and interrogated by US Immigration in Canada on my views on drones. My stance is known. Drone attacks must stop," he wrote on Twitter.
US authorities say they cannot give detailed comments on individual immigration cases due to privacy laws but a State Department official confirmed Khan had been delayed.
"We're aware that he was briefly delayed in Toronto before boarding his flight to the United States. The issue was resolved and Imran Khan is welcome in the United Sates," said the official.
Earlier this month, Khan led a march to northern Pakistan to protest the drone strikes, which have killed between 2,600 and 3,400 Pakistanis, according to the independent London-based Bureau of Investigative Journalism.
Most victims were reportedly militants but women and children have also been killed. Exact casualty figures are unclear since foreigners are rarely granted access to Pakistan's tribal areas, where several Islamist militant groups are based and many of the killings have been carried out.
Some Pakistanis say Khan is fanning anti-American sentiment to bolster his political career and criticise him for refusing to condemn atrocities by the Taliban or Pakistani army.
Others praise him for reaching out to the long-neglected population in Pakistan's northern tribal areas and say he is standing up for a war-ravaged population ignored by mainstream politicians.