Two American women will on Friday become the first female soldiers to graduate from the elite Ranger School combat leadership course, the military said.

Graduates of the training program are some of the toughest and most physically fit soldiers in the US Army, trained in airborne and air assault operations.

The women who graduate this week will still not be allowed to serve with the 75th Ranger Regiment, which has yet to lift its bar on female soldiers.

But women are increasingly being permitted into combat roles in other US Army units.

The notoriously challenging Ranger School welcomed women for the first time this year, following President Barack Obama's 2013 request that the Pentagon order all branches of the armed forces to open up ground combat roles to women by 2016.

"Congratulations to all of our new Rangers. Each Ranger School graduate has shown the physical and mental toughness to successfully lead organizations at any level," said Secretary of the Army John McHugh.

"This course has proven that every soldier, regardless of gender, can achieve his or her full potential."

Nineteen women began the rigorous 61-day training program in April but most were eliminated -- along with many men -- and one was sent back to attempt part of the course again.

"We owe soldiers the opportunity to serve successfully in any position where they are qualified and capable, and we continue to look for ways to select, train, and retain the best soldiers to meet our nation's needs," McHugh added.

- Half drop out in four days -

The intense program is divided into various phases that test soldiers' physical and mental toughness, some phases of which have to be repeated to pass.

Many candidates are weeded out in the first four days of punishing marches, navigation drills and physical fitness tests.

In all, students train some 20 hours per day, most of which is field instruction, with just over three hours set aside for sleep, the US Army association reports.

Students patrol some 200 miles (320 kilometers) and carry up to 90 pounds (40 kilos) of equipment.

The progress of the women has been closely monitored by the military community, where the idea of female forces in combat is still a divisive issue.

The exclusion of women from combat roles has been cited as a ceiling on female officers as it can in practice prevent them from reaching the highest military ranks where combat experience is considered an indispensable qualification.

Advocates point out that many women have already been exposed to the rigors of combat in recent foreign conflicts -- insurgencies with blurred front lines -- and fared well.

And five previously closed elite units have been opened up recently, including the Army Special Operations Command (Airborne), according to the Army Times.

Women make up about 15 percent of army personnel. Since the decision to open some combat positions to women, about 91,000 posts out of some 331,000 previously closed have been opened, according to the Defense Department.

The Army, Marine Corps, Air Force and Navy can still request for women to be barred from specific jobs. The door has yet to be opened for women in infantry combat roles.

Despite the opening up of combat roles to both sexes, military officials said last month that they do not expect huge numbers of women to take up those posts.