MEXICO CITY (AFP) - President Enrique Pena Nieto proposed on Tuesday (May 17) a constitutional reform to legalise same-sex marriage across Mexico following a Supreme Court ruling that opened the door to such unions.

"This way, equal marriage will be clear in our constitution," Pena Nieto said at an event marking the national day against homophobia.

Mexico City has authorised gay and lesbian marriages since 2009 and three of the nation's 31 states have followed suit. A fourth state, Campeche, has approved legislation but it has yet to be implemented.

The Supreme Court issued a landmark ruling last year declaring that it was unconstitutional for Mexican states to ban same-sex marriage.

While the "jurisprudence" issued by the court does not oblige states to change their laws, it requires courts to rule in favor of same-sex couples whose marriages were rejected.

Elsewhere in Latin America, Colombia became the fourth South American country to allow same-sex marriage when the constitutional court definitively legalised it last month.

Argentina was the first in South America to legalise same-sex marriage, in 2010, followed by Uruguay and Brazil three years later.