An Afghan walks in front of a house damaged in a blast during a wedding in Kandahar's Argandab district. (Allauddin Khan/Associated Press)

Canada is condemning a suicide attack on a wedding celebration in southern Afghanistan that left at least 40 people dead and dozens more injured.

The attack demonstrates the disregard insurgents have for Afghan civilians, Foreign Affairs Minister Lawrence Cannon said in a statement.

"On behalf of all Canadians, I extend my sincerest condolences to the families and friends of those who were killed and injured in this attack," he said.

The bomb blast late Wednesday almost completely flattened the outer wall of a compound in the Arghandab district of Kandahar, where male wedding guests had gathered for a meal. The windows and walls of the mud-brick dwellings were shattered and cracked.

Women guests at the party were in another compound that was not hit by the explosion, The Associated Press reported.

NATO said no service members from the alliance were involved or operating in the area at the time of the explosion, and U.S. military spokesman Col. Wayne Shanks said the deaths were not the result of an airstrike.

The International Security Assistance said Afghan police secured the site after the attack, adding that Afghan authorities are working to determine the cause of the blast.

A Taliban spokesman denied responsibility for the attack.

At a news conference in Kandahar city, provincial Gov. Tooryalai Wesa held up a chunk of metal he said was from a suicide bomb used in the attack, and rejected the insurgents' claim of innocence.

"The Taliban are doing two things at once," Wesa said. "On one side they target people who are in favour of the government, then at the same time they don't want people to know their real face."

Kandahar mission facing delays

A military and civilian campaign against the Taliban began in the Kandahar region this spring, and had been expected to ramp up in June and largely conclude by August. It will now probably stretch far into the fall, U.S. General Stanley McChrystal said Thursday.

The operation to secure the Kandahar region will unfold more slowly and last longer than the military had planned, said McChrystal, the top commander in Afghanistan

"It will take a number of months for this to play out, but I don't think that's necessarily a bad thing," McChrystal told reporters on the sidelines of a NATO meeting taking stock of the war.

"I think it's more important that we get it right than we get it fast," he said.

Kandahar is key to the success of U.S. President Barack Obama's revamped war strategy, which focuses on turning local allegiances against the Taliban and toward the U.S.-backed central government.

With files from The Associated Press and The Canadian Press