"We are aware that he was detained briefly in Toronto before boarding his flight to the United States," acting State Department spokesman Mark Toner told reporters when asked about Khan's questioning last week.

But he disputed allegations that Khan was held and interrogated for hours as he tried to board a New York-bound plane from the Canadian city.

"My understanding is that the issue was quickly resolved and that he traveled into the United States, and certainly he is welcome in the United States," Toner said.

Khan was headed to New York to attend a fundraiser organized by his party.

"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.

Khan argues drone strikes are illegal and counterproductive and earlier this month he led thousands of supporters -- and a group of American peace activists -- on a march to the edge of Swat to protest against them.

Toner said he did not know "the specifics" of why Khan was held, but added: "I would just say that we certainly don't want any legitimate traveler delayed for no reason."

Copyright AFP (Agence France-Presse), 2012