"We affirm the inherent dignity of each individual as expressed in the Universal Declaration of Human Rights," the letter reads.

"Respect for these fundamental rights ... obliges governments to protect all citizens from violence and discrimination and to ensure they enjoy equal opportunities."

LGBTI rights are a sensitive issue in Poland.

During this year's presidential campaign President Andrzej Duda told a rally "one tries to make us believe that LGBT are people but it is simply an ideology".

Domestic and international outrage has also been sparked by the non-binding resolutions "against LGBT ideology" adopted by dozens of local governments in Poland.

Human rights "are not an ideology - they are universal," US Ambassador to Poland Georgette Mosbacher, one of the signatories of the letter, wrote on Twitter on Sunday evening.

For conservative voters and lawmakers in Poland, campaigning for LGBTI rights is equivalent to an attack on family values.

They also use the term "LGBT ideology" to mean support for abortion or changing traditional roles in the family.

Poland does not allow marriages or civil unions for same-sex couples, though some surveys indicate the majority of the population would support the latter.

In response to the letter, Poland's Deputy Foreign Minister Pawel Jablonski said the country fully agreed with its message of inclusivity.

"Every human being enjoys equal level of protection under the Polish law. No one should be discriminated against in political, social or economic life - for any reason whatsoever," Jablonski tweeted.

But, he added, "We also remind that according to the Polish Constitution, marriage is a union of a woman and a man.

"Statement of this fact (also in resolutions of local authorities) is not discrimination - it is a sign of respect for the rule of law, and for the values that Poles hold dear."

Australian Associated Press