Rice conveyed her "disappointment" over the withdrawal to her Canadian counterpart Pierre Pettigrew when they met at the sidelines of a London meeting on Palestinian reforms, a department official said.

The top US diplomat was scheduled to visit Canada in the second week of April, but her spokesman Richard Boucher said "the schedules didn't work out".

"We are looking for a date when we can make that happen," he said, without confirming or denying reports that the cancellation was due to problems over missile defense.

But, speaking on condition of anonymity, a US official travelling with Rice to the London meeting said the trip's cancellation was "in part" due to Canada's refusal to cooperate on missile defense.

Canada said last week it would not participate in US missile defense shield plans after mulling its stand on the issue for more than a year.

Relations between the two neighbors had appeared to warm of late after cooling with the US-led invasion of Iraq in early 2003, in which Canada had also declined to participate.

Canada's Liberal Prime Minister Paul Martin leads a minority government and his party's lawmakers had lobbied him not to take part in the program, which is highly unpopular in Canada, particularly in Quebec.

Rice was supposed to visit Canada after an upcoming summit between Martin, US President George W. Bush and Mexican President Vicente Fox on March 23.

The State Department has not given any possible new dates for Rice's visit to Canada.

A department official, speaking on condition of anonymity, said Rice and Pettigrew met during a reception on the sidelines of the Palestinian meeting and spoke about the missile defense issue.

"He explained the political process and she explained her disappointment but both came out saying they are committed to their defense relationship and working together wherever we can," the official said.

"She said she looked forward to visiting (Canada) and he said he looked forward to welcoming her," the official added, indicating that the two countries had put the issue behind them.

White House spokesman Scott McClellan also denied in Washington that Canada's announcement on the missile shield would jar cooperation between two giant neighbors, often referred to as joint custodians of the world's longest undefended border.

"We're going to continue to work with Canada and cooperate with them on shared defense priorities," McClellan said. "We have been cooperating on defense issues in the past and we will in the future."

McClellan did not, however, scotch reports that Bush expressed displeasure at the decision by declining to return a call from Martin to explain it: "I'll have to check on that, I don't know if that's the case," he told AFP.

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