Iran's supreme leader says that the downing of a civilian airliner leaving Tehran was a bitter accident that saddened Iran and made its enemies happy.

In a rare sermon at Friday prayers in Tehran, Ayatollah Ali Khamenei says Iran's international rivals have seized on the crash to question the country and its armed forces.

The crash of a Ukraine International Airlines jet after it was hit by at least one surface-to-air missile last week killed 176 people, including 138 bound for Canada.

Khamenei also attacks the United States for its killing of a top Iranian general, which was a factor in putting Iranian air defences on a hair-trigger, and calls President Donald Trump a clown.

In London on Thursday, Canada called a meeting of most of the countries affected and laid out demands for Iran's co-operation with the investigation, punishing those responsible, and compensating victims' families.

Prime Minister Justin Trudeau has scheduled a news conference in Ottawa this morning to report on Canada's efforts since the strike.

--With files from The Associated Press