The first three women in history have been appointed to serve as infantry marines in the US military.

An infantry battalion at Camp LeJeune in North Carolina has welcomed the three women who will serve in the 1st Battalion, 8th Marine Regiment as rifleman, machine gun and mortar marine, said 1st Lieutenant John McCombs, a marines spokesman.

Their names have not been released as they become acclimatised to their unit, as reported by the Marine Corps Times.

There are already three women in leadership roles in the unit to help with the transition, said Mr McCombs.

Defense secretary Ash Carter opened all combat positions to women as of January 2016, ending a century-old battle for women to serve alongside men.

The Marine Corps was the only service branch to rebuff the directive, and asked for an exception when it came to women serving in the most dangerous commando posts. The request was rejected.

President-elect Donald Trump said he would allow women to serve as "they’re really into it" but has previously questioned men and women working together.

In 2013 he tweeted a suggestion that the big number of sexual assaults in the army were a result of women and men simply serving together. He stood by his remark when he was asked about it in September by NBC.

Mr Trump’s incoming defense secretary, retired general James Mattis, has not been keen to see women serve and said it would lead to “reduced standards”.

“When you have to reduce standards — as you would have to do, you would have to do it — and when you would mix, you know, when you mix eros, when you mix affection for one another that could be manifested sexually, I don't care if you go anywhere in history, you will not find where this has worked. Never has this worked,” Mr Mattis said in 2015.