Franklin Graham has issued a rare rebuke of Donald Trump's administration, slamming a policy that separates children from their parents at the US-Mexico border as 'disgraceful'.

The prominent evangelical leader, and son of famous evangelist Billy Graham, has been one of Trump's most ardent supporters.

Figures from Reuters suggest 1,800 families were separated at the US–Mexico border from October 2016 to February 2018.

Graham joined the growing criticism at the policy but said previous administrations were to blame.

'It's disgraceful, and it's terrible to see families ripped apart and I don't support that one bit,' he told CBN.

However it is Trump's attorney general Jeff Sessions who last month implemented a 'zero tolerance' policy towards those caught entering the US illegally. All those caught would be criminally charged under the new approach, which generally leads to children being separated from their parents.

But Graham said: 'I blame politicians for the last 20, 30 years that have allowed this to escalate to the point where it is today.'

He told CBN: 'We are a country of a laws, laws need to be obeyed, no question about that, but the situation we have today as a result of our lawmakers in Washington over generations ignoring this.'

The US Catholic bishops began their annual spring assembly with a more outright rebuke at Trump's immigration policies.

'The latest developments are consistent with the sort of cardiosclerosis that has begun in our country, and it concerns across-the-board life issues,' said Cardinal Joseph Tobin of Newark, New Jersey. The bishops are also considering travelling to the border detention camps where children are being held 'as a sign of our pastoral concern and protest against this hardening of the American heart.'

'And the invitation is to harden our hearts further,' said Cardinal Tobin, who suggested the idea.

A statement from Cardinal Daniel DiNardo, president of the US conference of Catholic bishops, said: 'At its core, asylum is an instrument to preserve the right to life. The Attorney General's recent decision elicits deep concern because it potentially strips asylum from many women who lack adequate protection. These vulnerable women will now face return to the extreme dangers of domestic violence in their home country.'

'Families are the foundational element of our society and they must be able to stay together,' DiNardo said.

'While protecting our borders is important, we can and must do better as a government, and as a society, to find other ways to ensure that safety. Separating babies from their mothers is not the answer and is immoral.'

A US Customs and Border Protection (CBP) official testified last month to Congress that between May 6 and May 19, 658 children were separated from 638 parents because of the stepped-up prosecutions. That brings the total of officially acknowledged separations to more than 2,400, though that does not include recent weeks or the period from March 1 to May 6.

Immigration and child advocates, Democratic lawmakers and the United Nations have all condemned the practice of separating families at the border, but the administration has defended its actions saying it is protecting children and making clear that illegal border crossers will be prosecuted regardless of their family circumstances.

In most of the 1,768 cases of families separated by border agents between October 2016 and February, children were removed from parents for medical reasons or because of security concerns, the official said, citing examples such as parents needing hospitalization or officials discovering the parent had a criminal record either in the United States or in their home country.

In 237 cases, an official said, children were removed because border agents suspected adults were falsely posing as the parents of minors in their charge.

The period for which statistics were provided included the final three months of the Obama administration in 2016, but the official could not say whether any of the separations occurred then.