The Slovenian-born parents of US First Lady Melania Trump have become United States citizens at a naturalisation ceremony in New York.

President Donald Trump's in-laws, Viktor and Amalija Knavs, took the oath of citizenship, their immigration lawyer Michael 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.

Mr Trump has taken a hard line on immigration policy, criticising so-called chain migration that allows naturalised US citizens to sponsor close relatives for permanent residency.

He 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.

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