United States defended its use of a Trump-era health law, which has been reportedly used to deport a series of undocumented migrants, according to US media reports.

The news comes as the country is on a track to lift restrictions from the southern and northern borders for nationals traveling from Mexico or Canada.

White House press secretary Jen Psaki, when asked about the policy on permitting fully vaccinated individuals foreign nationals to seek asylum, said on Wednesday that the law, which has been dubbed as Title 42, "remains in place".

Foreign nationals who will be carrying "proper documentation" and will be fully vaccinated against COVID-19 will be permitted to enter United States.

The law, Title 42, was introduced by former United States President Donald Trump in March last year as concerns were growing around the COVID-19 pandemic. Under the legislation, more than 1 million migrants have been expelled since it was put in place, according to US media reports citing Customs and Border Protection.

In practice, Title 42 means that anyone caught crossing the border illegally, namely without a visa to enter or without going through a formal border checkpoint, can be deported immediately without being offered the opportunity to claim asylum in the United States.

All migrants who entered the United States -- with or without documentation-- before the law had been put in place were given an opportunity to seek asylum.

A senior administration official said on Tuesday, "The Title 42 restrictions are really about protecting the migrants themselves, the DHS workforce and local communities", according to reports from CNBC. 

The official added, "There is a strong public health basis, for the moment, for continuing with the Title 42 restrictions."

The United States Centers for Disease Control and Prevention said in August that the law would continue to be in place until there is no longer a danger. The federal body added that children are exempted from the health law.