United Airlines has been criticised on social media after it barred two girls from flying for wearing leggings.

The incident happened on a flight from Denver to Minneapolis on Sunday morning, BBC quoted activist Shannon Watts as saying.

A United gate agent was "forcing" the girls, one of them aged 10, to change their clothes or wear dresses over the leggings, Watts tweeted.

United said the girls were travelling on a ticket that had a dress code.

They were "United pass travellers", which are tickets for company employees or eligible dependents, it explained in a Twitter exchange on the issue.

Shannon Watts, founder of the group Moms Demand Action for gun reforms, tweeted about what happened to five girls when they tried to board a flight at Denver airport.

She said three of the girls were allowed to fly after putting dresses over the top of their clothing, but two were prevented from boarding.

She slammed the airliner for its actions, asking: "Since when does United police women's clothing?".

Although United has not officially commented on the incident, it did respond on Twitter by explaining the dress code requirement of its United pass travellers.

Watts' tweets have been shared and responded to by thousands of users, including actress and activist Patricia Arquette.