The US military will open up all positions, including frontline combat roles, to women, defence secretary Ashton Carter has announced in a sweeping move that ends centuries of men-only tradition.

The historic step trumps a Marine Corps call for a partial exemption after it claimed mixed-gender combat units were not as effective as male groups.

“There will be no exceptions,” Mr Carter said.

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Captain Kristen Griest and 1st Lieutenant Shaye Haver have made US military history. Photo: ABC

While acknowledging that on average physical differences exist between sexes, Mr Carter said plenty of women could meet the military’s rigorous physical standards for combat roles — just as there are some men who cannot.

“As long as they qualify and meet the standards, women will … be allowed to drive tanks, fire mortars, and lead infantry soldiers into combat,” he said.

“They’ll be able to serve as Army rangers and green berets, Navy SEALS, Marine Corps infantry, Air Force parajumpers and everything else that was previously open only to men.”

Given the tough physical standards, Mr Carter stressed that equal opportunity would probably not translate to equal participation of men and women in all roles.

“There must be no quotas or perception thereof,” he said.

There are currently around 1.38 million people in the US military, and about 15.5 per cent of them are women.

The move comes nearly three years after the Pentagon first eliminated its ban on women serving in front-line combat roles and began a process that would let women compete for 220,000 additional military jobs.

Then-defence secretary Leon Panetta lifted a ban in force in 2013 on women in front-line combat roles, a restriction seen as increasingly out of place during a decade of war in Iraq and Afghanistan in which women were increasingly in harm’s way.

Women represented about two per cent of US casualties in Iraq and Afghanistan, with around 300,000 deploying to the conflict zones.