The calls for FIFA to change its turf policies are growing louder -- and taking on a more serious tone.

In a letter delivered to FIFA, which runs the men's and women's World Cups, 13 U.S. senators urge the organization to switch from synthetic to natural grass for its playing fields at the 2015 Women's World Cup.

The letter also criticizes the organization for making women play on synthetic fields in the first place. Men are given natural grass playing fields for all of their major tournaments.

The differing treatment, the senators argue, is a serious gender-equality issue that the organization must address.

"We are deeply concerned with FIFA's treatment of these players," write the senators in the letter. "We urge you to begin good faith negotiations with these athletes, free of retaliation and with the equal treatment that they deserve."

The senators -- 12 Democrats and one Republican -- join a growing chorus of advocates for the female soccer players, including Kobe Bryant and former women's soccer star Mia Hamm.

FIFA has not given any indication that it will change the policy. One speculation regarding its resistance is that using natural grass fields would represent a significant increase in cost due to the maintenance the fields would receive.

FIFA is also being targeted by more than 40 international women in a lawsuit alleging gender discrimination.

Related Story: Is Synthetic Turf Giving Athletes Cancer?

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