A coalition of 20 US states announced on Monday they will sue Donald Trump's administration over its recent decision to remove legal limits on how long migrant children can be detained.

"With this rule, the Trump administration is paving the way for ICE to imprison innocent children for indefinite periods of time," Massachusetts Attorney General Maura Healey said in a statement.

The Department of Homeland Security last week said it was terminating the 1997 Flores Settlement Agreement, a legal ruling that said the government could hold migrant children in detention for no more than 20 days.

A new policy, to be implemented in less than 60 days, will allow children and their families to be detained for an unlimited time.

Human rights groups have also criticised the Trump administration's new rule and vowed to oppose it in court.