By Anna Mehler Paperny

TORONTO (Reuters) -Canada this week updated its travel advisory for the United States, warning travelers with an "X" gender marker on their passports that they may not be allowed to enter the country.

"While the Government of Canada issues passports with a 'X' gender identifier, it cannot guarantee your entry or transit through other countries," the new warning reads.

The warning says travelers may still be asked to provide sex or gender information as either male or female.

Canada is one of a number of countries that allows citizens who do not identify as either female or male to list their gender with an "X" on passports, rather than an "F" or "M".

U.S. Customs and Border Protection said in an e-mailed statement that foreign travelers' gender as indicated on their passport and their personal beliefs about sexuality do not render them inadmissible to the United States.

The United States itself has also issued passports with an "X" under a policy begun under the administration of President Joe Biden. President Donald Trump issued an executive order in January recognising only two biological sexes, including on government-issued identification documents.

A federal judge's order barred the U.S. State Department from enforcing Trump's ban blocking the issuance of passports that reflect the gender identities of transgender and nonbinary Americans. The Trump administration has appealed to the Supreme Court.

Reuters reported in July that Canada had drafted a similar warning but had not published it at the time. Global Affairs Canada did not immediately respond to questions as to why it was issuing this warning now.

As of July, Global Affairs Canada was not aware of any Canadian denied entry to the U.S. because of the "X" gender identifier on their passport, a spokesperson said.

(Reporting by Anna Mehler Paperny in Toronto; Editing by Caroline Stauffer)