The Slovenian-born parents of Melania Trump became US citizens at a naturalisation ceremony in New York on Thursday, reportedly taking advantage of a family reunification programme President Donald Trump has vehemently denounced.

Mr Trump's in-laws Viktor and Amalija Knavs took the oath of citizenship, their immigration lawyer Michael Wildes confirmed to AFP.

Asked by the New York Times if they had obtained citizenship under a programme derisively branded "chain migration" by the president, Mr Wildes replied: "I suppose."

He said chain migration, which allows naturalised US citizens to sponsor close relatives for permanent residency, was a "dirtier" way of characterising what he called "a bedrock of our immigration process when it comes to family reunification."

Trump has taken a hardline on immigration policy, criticising so-called chain migration.