New York City has warned landlords, employers and businesses they could be running afoul of the law by purposely calling a transgender woman "him" or "Mr." when she prefers a female title and pronoun, or by barring her from using a women's restroom.

New guidelines detail the legal protections of transgender and gender-nonconforming New Yorkers and what constitutes discrimination under the city's Human Rights Law, the New York City Commission on Human Rights said on Monday.

Some 25,000 transgender and gender nonconforming people live in New York City, where discrimination based on gender identity and expression has been illegal since 2002.

"Today's new guidelines strengthen those laws by ensuring that every transgender and gender nonconforming person in New York receives the dignity and respect they deserve," said Mayor Bill de Blasio.

The guidelines said dress codes requiring men to wear ties or women to wear skirts are discriminatory.

Failing to provide employee health benefits for gender-affirming care or failing to accommodate people undergoing gender transition, such as medical appointments, could violate the law as well, they said.

"It's a huge step forward and really catapults New York to the forefront of the struggle for transgender rights," said Michael Silverman, executive director of the Transgender Legal Defense and Education Fund.