Donald Trump is being lambasted for describing refugees the Obama administration agreed to resettle from Australian detention camps on Pacific islands as “illegal immigrants”.

His predecessor pledged to house up to 1,250 of the most vulnerable asylum seekers held in "inhumane" centres on Papua New Guinea and Nauru, while Australia agreed to resettle refugees from Central America.

Mr Trump called the agreement struck last year a “dumb deal” on Twitter in an unprecedented public attack on one of America’s closest allies.

Leaked Nauru report highlights ‘appalling’ refugee conditions

The President had reportedly aired his grievances in an angry phone call with Malcolm Turnbull on Wednesday, when the Australian Prime Minister attempted to confirm whether the arrangement still held.

Describing it as the “worst deal ever”, Mr Trump accused his ally of attempting to export the “next Boston bombers”, the Washington Post reported.

He followed up the call by tweeting: “Do you believe it? The Obama Administration agreed to take thousands of illegal immigrants from Australia. Why? I will study this dumb deal!”

Critics quickly pointed out that the men, women and children to be resettled could not be illegal when their transfers had been organised and facilitated by the US government.

To be defined as an illegal immigrant, people must have violated a country’s laws by either crossing the border irregularly or overstaying their visas.

The vast majority of migrants arriving in Europe in the ongoing refugee crisis initially fulfil the definition but then become asylum seekers by exercising their right under the 1951 Refugee Convention to apply for international protection.

Australians join a rally organised by the Refugee Action Coalition calling for the closure of the Manus and Nauru detention centres (Getty)

The US agreement with Australia, by contrast, sees men, women and children who have already been vetted by the Department of Homeland Security legally transferred into the country by the American government.

The United Nations and humanitarian groups have raised concern over Australia’s camps in Papua New Guinea and Nauru, where all asylum seekers attempted to reach the country by sea are held indefinitely as part of the controversial “stop the boats” policy.

Refugees have set themselves on fire in protest at dire conditions at the processing centres, where physical, mental and sexual abuse has been reported.

Most are Muslims from the Middle East, Africa and Asia, including the persecuted Rohingya minority, meaning their arrival would contravene Mr Trump’s executive order suspending the US refugee programme and immigration from seven Muslim-majority “countries of concern”.

He claimed the move would prevent “bad dudes” coming to the US, despite the fact countries linked to previous terror attacks were not on the list, and warnings the “Muslim ban” would fuel propaganda efforts by Isis and other jihadi groups.

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Despite the President’s statements, the White House and the US Embassy in Australia both said Mr Trump would honour the agreement, while Mr Turnbull reiterated that he believed the deal stood in media appearances following the tweet.

Australia has refused to resettle any refugee arriving by boat since July 2013, whatever their circumstances, instead paying Pacific nations to detain them while searching for other countries to take the arrivals permanently.

After the transfer deal was struck with the US in November, Mr Turnbull said: “Our priority is the resettlement of woman, children and families.

“This will be an orderly process. It will take time. It will not be rushed.”