(Adds comments by Canadian opposition leader)

KABUL, July 4 (Reuters) - Six Canadian soldiers were killed in southern Afghanistan on Wednesday when their armored vehicle hit a roadside bomb, the Canadian government said, in what was the deadliest attack on NATO forces this month in the country.

The Taliban claimed responsibility for the attack, the deadliest since April, when six Canadian soldiers were killed in a similar incident. It brings the total number of foreign troops killed in action in Afghanistan to more than 70 this year.

Afghanistan is going through one of its bloodiest phases of violence since U.S.-led troops overthrew the Taliban's radical Islamic government in 2001. The Taliban are strongest in southern Afghanistan.

Ottawa said the six men and a local Afghan interpreter died when their armored vehicle hit the device in the Panjwai district about 20 km (13 miles) southwest of the southern city of Kandahar, which is home to Canada's 2,600-strong mission.

"Clearly, they have managed to kill six great young Canadians today, which is an absolute tragedy," Canadian Brigadier-General Tim Grant told a televised news conference in Kandahar.

So far, 66 Canadian soldiers and a diplomat have died since Ottawa sent troops to Afghanistan in late 2002 as part of the U.S.-led war on terror.

The Taliban rely largely on suicide attacks and roadside bomb blasts in their campaign against foreign troops and the government.

The blast occurred on a gravel road as the troops were returning in a convoy of 12 vehicles from a joint operation with the Afghan national army.

"As with every attack we will look at what has happened and will decide at that time if there is something we need to do to increase the protection for our soldiers," said Grant, adding that Canadian troops were comfortable in the Panjwai district.

"We're not perfect and we do miss some (roadside bombs) as was seen today. But the battle against the Taliban and their choice of weapons ... is successful," he said.

Late last month three Canadians died in Panjwai district when their unarmored supply vehicle triggered a roadside bomb in what was supposed to be a secure area.

The latest deaths will only increase doubts among Canadians about the wisdom of the mission, which is due to end in February 2009. Critics say the force is focused too much on fighting and not on rebuilding the country.

Prime Minister Stephen Harper has pledged to put any military involvement after February 2009 to a vote in Parliament, where his Conservative government has only a minority of seats.

Stephane Dion, leader of the official opposition Liberals, said Parliament would not back any such move.

"This consensus will never exist ... the prime minister should say to NATO right away that the combat mission will end in February 2009," he told a news conference.

The Liberals and the Bloc Quebecois say the mission should end on time. The minority New Democrats, the third opposition party, want Canada's troops out immediately.

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