Canada allowed the U.S. National Security Agency (NSA) to conduct widespread surveillance during the 2010 Group of 20 summit in Toronto, according to a media report that cited documents from former NSA contractor Edward Snowden.
The report by the Canadian Broadcasting Corp is the latest potential embarrassment for the NSA as a result of Snowden's leaks, although it remains unclear precisely what information the agency was looking for during the summit.
Snowden has already revealed the agency spied on close allies such as Germany and Brazil, prompting heated diplomatic spats with Washington.
The CBC report, first aired late on Wednesday, cited briefing notes it said showed the United States turned its Ottawa embassy into a security command post during a six-day spying operation by the top-secret U.S. agency as President Barack Obama and other world leaders met that June.
Reuters has not seen the documents and cannot verify their authenticity. One of the bylines on the CBC report was Glenn Greenwald, the U.S. journalist who has worked with Snowden on several other NSA stories.
CBC said the operation was no secret to Canadian authorities and it quoted an NSA briefing note describing the operation as "closely coordinated with the Canadian partner".
The Canadian equivalent of the NSA is the Communications Security Establishment Canada, or CSEC.
U.S. authorities declined to comment specifically on the report.
"As a matter of policy we have made clear that the United States gathers foreign intelligence of the type gathered by all nations," said Caitlin Hayden, spokeswoman for the White House National Security Council.
CBC said the documents did not reveal the targets of the NSA operation, but described part of the U.S. eavesdropping agency's mandate at the Toronto summit as "providing support to policymakers".
A spokesman for Canadian Prime Minister Stephen Harper declined to comment on the allegations in the report, but said security organizations were subject to independent oversight.
CSEC, which has a very low public profile, employs about 2,000 people. It is part of the so-called Five Eyes intelligence-sharing network that also includes the United States, Britain, New Zealand and Australia.
Last month, Brazil angrily demanded an explanation for media reports which said CSEC agents had targeted its mines and energy industry.
CSEC head John Forster, pressed about the CBC report at a meeting of the House of Commons defense committee, declined to comment on the specifics of Canada's intelligence operations, but appeared to play down the idea his agency had played an active role during the G20 summit.
"Under law, CSEC cannot target Canadians anywhere in the world or anyone in Canada, including visitors. I cannot ask my international partners to do anything that I am not allowed by law to do," he told legislators on Thursday.
Forster's comments left open the possibility that the NSA had requested help from CSEC.
Canada's Defense Ministry, which has overall responsibility for CSEC, says it needs C$461 million ($435 million) to run CSEC in 2013/14, up from C$439 million in 2012/13.
Forster said CSEC needed an extra C$44 million over five years to upgrade a top-secret communications network, adding there was "no direct relationship" between that request and the agency's activities in Brazil. He did not give more details.
The British Columbia Civil Liberties Association and the OpenMedia.ca lobby group announced a lawsuit against CSEC last month, alleging that activities such as spying in Brazil were illegal and unconstitutional.
OpenMedia.ca executive director Steve Anderson said Canadians watching the CBC report would "be shocked to discover just how secretive, expensive, and out-of-control our government's spying activities are".