WASHINGTON (REUTERS) - The United States will no longer grant diplomatic visas to the same-sex domestic partners of foreign diplomats, requiring that they be legally married to obtain them, in a reversal of Obama-era guidelines, senior US officials said on Tuesday (Oct 2).

The new policy was circulated in a United Nations memo last month and took effect on Monday.

The UN memo, which was published online, states that the US State Department will not issue a G-4 visa, which are for employees of international organisations and their immediate family members, to same-sex domestic partners.

Instead, same-sex partners of diplomats based in the United States will have to present proof of marriage to be eligible for such visas.

The new rules reverse a 2009 policy instituted by former Secretary of State Hillary Clinton, which defined "family" of foreign diplomats as including same-sex domestic partners, thus making them eligible for diplomatic visas.

Diplomats currently in the United States will be able to fulfill the new requirement by getting legally married in the United States, a senior Trump administration official said in a briefing with reporters on Tuesday. According to the UN memo, they must submit proof of marriage by Dec 31, or leave the country within 30 days.

The senior Trump administration official said the United States will have a process to recognise same-sex domestic partners of foreign diplomats from countries where same-sex marriage is not legal, but which recognise American domestic partners.

About 105 families in the United States will be affected by the policy change, and 55 of them are with international organisations. A second senior US official said "very few" of the affected families are from countries where same-sex marriage is illegal.

"We are aware of the policy change in the US, which was conveyed to us in July, and are trying to abide by it," United Nations spokesman Farhan Haq said.

Haq said the United Nations believes the policy affects 10 staff members who would need to provide marriage certificates before the end of this year.

US officials told reporters that the change was meant to align US policy towards foreign diplomats with State Department policy towards American diplomats posted abroad.

"US diplomats as of yesterday have to be legally married in order to get this sort of derivative diplomatic status when they go overseas, so these changes are to mirror what US policy now is," one of the senior administration officials said, speaking on condition of anonymity.

The US Supreme Court in 2015 ruled that the Constitution provides same-sex couples the right to marry, and internal State Department guidelines have changed to reflect that, officials said.

The new US policy has drawn criticism from gay rights advocates who say it is not accommodating toward diplomats who come from countries that are hostile to same-sex unions. Twenty-six countries allow gay marriage, according to a 2017 tally by the Pew Research Centre.