Human rights lawyers and activists have sued the government of Eswatini for making a secretive deal with President Donald Trump's administration to accept third-country deportees from the US, which they claim was unconstitutional. 

The case was due be heard at the High Court of Eswatini on Friday, but was postponed until September 25 because the government did not file response papers, the lead applicant told Reuters outside the court on Friday. 

Eswatini's Attorney General Sifiso Khumalo said in a text message that the case had no legal basis. "It's a frivolous legal application," he wrote.

In July the US deported five individuals from Vietnam, Jamaica, Laos, Cuba and Yemen to the Southern African country. All were convicted felons, and Eswatini says it is holding them in solitary confinement until they can be repatriated.

The applicants in the case, led by the Eswatini Litigation Centre, say that the agreement made with the US was illegal because it was not submitted to parliament for approval and the terms were not disclosed.

They also say they do not know the condition of the deportees because no one has been allowed access to them.

"We want the executive to be held accountable, we want transparency dealing with matters of state importance, (and) respect for the rights of all individuals who are in Eswatini regardless of who they may be," said lead applicant and lawyer Mzwandile Masuku. 

Eswatini, an absolute monarchy ruled by King Mswati III, has previously said the deportees pose no threat and that the agreement was based purely on its good relations with Washington.

The International Organization for Migration told Reuters it had received a request from Eswatini to provide "post-arrival assistance" for the deportees, but did not say whether it would accept or what that might entail. 

"We are discussing with Government of Eswatini, their request," a spokesperson for the UN agency said on Thursday. 

Trump aims to deport millions of immigrants who entered the US illegally and his administration has sought to increase removals to third countries.