WASHINGTON (AFP) - The United States government has announced that married same-sex couples may now apply for "green card" residency permits just as heterosexual married couples can, in the latest move to end discrimination against gay unions.

Homeland Security Director Janet Napolitano announced the move on Monday.

It stems from last week's landmark Supreme Court ruling that extends federal rights and benefits to gay couples who wed in states that recognise such unions.

After the court ruling, President Barack Obama directed federal agencies to ensure it is implemented smoothly, Ms Napolitano said in a statement.

She added: "I have directed US Citizenship and Immigration Services (USCIS) to review immigration petitions filed on behalf of a same-sex spouse in the same manner as those filed on behalf of an opposite-sex spouse."

One such petition by a gay couple married in New York was approved immediately after the Supreme Court's ruling.

It was a race against the clock for the two men because the non-US resident among them, an undocumented alien from Colombia, was on the verge of being deported by an immigration court.

The judge in that case acknowledged the landmark nature of the Colombian's status in the face of the now-overturned Defence of Marriage Act (Doma), said attorney Lavi Soloway, co-founder of the Doma Project, which has been defending 70 couples in similar situations.

"She understood the meaning of the case, and that meant that the couple before them was the first married same-sex couple in the United States that would be viewed as a married couple by a federal government agency," Mr Soloway said.

Twelve states and the capital city Washington, D.C. allow gay marriage.

After a second Supreme Court ruling on Wednesday, California resumed recognising same-sex marriage.