The US military will inevitably have to lower its physical requirements so more women can fill infantry and commando roles, a top general said Friday.

The comments from John Kelly, a four-star Marine general who is retiring from the head of the military's Southern Command, came just over a month after Defense Secretary Ashton Carter ordered all jobs in the military to be opened up to women, with no exceptions.

As he announced the order, Carter stressed that physical standards would not be lowered to ensure females qualified for a particular role.

The Marine Corps had spoken out against women serving in some combat roles, saying they were more likely to be injured than their male colleagues and claiming that mixed-gender combat units were not as effective as male groups.

"Right now they say they are not going to change any standards," Kelly told reporters at the Pentagon.

But "there will be great pressure" to do so he said, because a few years from now commanders will want to know why more women haven't qualified for certain positions.

"If we don't change standards it will be very, very difficult to have any real numbers come into the infantry," he said.

"The pressure for the generals that are to come (will be) to lower standards -- that is the only way it will work," he added.

Obama's administration in 2013 asked for all combat positions to be open to women by 2016, including the infantry, artillery, armor and special forces.

But Obama gave the Pentagon the opportunity to request exceptions, provided these were justified by operational constraints.

The Marine Corps asked for such an exemption, and was overruled by Carter.