The Slovenian-born parents of US First Lady Melania Trump became US citizens at a naturalization ceremony in New York on Thursday, their immigration lawyer Michael Wildes confirmed to AFP.

President Donald Trump's in-laws, Viktor and Amalija Knavs, took the oath of citizenship, Wildes said.

He did not specify how long it had taken the Knavs to complete the citizenship process, nor whether the 48-year-old First Lady had sponsored their permanent residency.

The parents of US first lady Melania Trump, Viktor and Amalija Knavs, await the start of the official arrival ceremony held by US President Donald Trump and Mrs Trump for French President Emmanuel Macron and his wife Brigitte Macron on the South Lawn of the White House in Washington, US on April 24, 2018. Photo: Reuters

Trump has taken a hardline on immigration policy, criticizing so-called chain migration that allows naturalized US citizens to sponsor close relatives for permanent residency.

The Republican president argues that the system steals jobs from Americans and threatens national security, calling for a merit-based system that preferences more educated, English-speaking professionals.

Viktor Knavs, a car salesman in Slovenia, and Amalija, who worked in a textile factory, are over 70 years old, retired and pass much of the year in the United States, where they regularly spend time with their daughter and grandson Barron.