First lady Melania Trump's parents have become US citizens after taking the citizenship oath during a ceremony on Thursday.

A lawyer for Viktor and Amalija Knavs revealed the Slovenian couple, who had already been living in the US as permanent residents, took the citizenship oath in New York City.

They came and went from the ceremony at a Manhattan federal building flanked by Homeland Security police.

There has been speculation that the President Donald Trump's in-laws may have benefited from the “chain migration” process that he has claimed endangers Americans and vowed to restrict.

According to experts, there are two main ways the First Lady's parents could have achieved citizenship status - either through sponsorship from their daughter or an employer.