Women should be included in the US military draft, a federal judge in Texas has ruled, in a case brought by a men's rights group.

Judge Gray Miller in the southern district of Texas said it was unconstitutional to only apply the draft to men, because women can now serve in combat roles too.

The US military has not enforced conscription in more than 40 years, but all American male teenagers are still required to register when they turn 18 in case a draft is reinstated. They remain eligible until they are 25 years old.

Those who fails to register can face penalties which range in severity from fines, imprisonment or being denied services like federal student loans.

Judge Miller said the Supreme Court's 1981 ruling excluding women from the draft was "justified" because women were not allowed to serve in combat roles at the time.

The Pentagon abolished those restrictions in 2015, meaning women can now serve in any military role.