The UN human rights office is calling on the Trump administration to "immediately halt" its policy of separating children from their parents after crossing the U.S. border with Mexico.

Rights office spokesperson Ravina Shamdasani scolded the U.S. for its "zero tolerance" policy in which some families have been separated after migration across the border.

The rights office said in a statement that "children should never be detained for reasons related to their own or their parents' migration status. Detention is never in the best interests of the child and always constitutes a child rights violation."

Shamdasani told a UN briefing Tuesday in Geneva that the practice of separating families amounted to an "arbitrary and unlawful" interference in family life, calling it a "serious violation" of the rights of children.

The admonition comes as President Donald Trump again on Tuesday blamed Democrats for the need to separate children, without precisely explaining why. It is a stance he first took on social media late last month.

Separating families at the Border is the fault of bad legislation passed by the Democrats. Border Security laws should be changed but the Dems can’t get their act together! Started the Wall. —@realDonaldTrump

Blatantly false. This is a direct result of the <a href="https://twitter.com/realDonaldTrump?ref_src=twsrc%5Etfw">@realDonaldTrump</a> administration’s policies. Only he can and MUST end this barbaric policy. <a href="https://t.co/JFNaYTy1N8">https://t.co/JFNaYTy1N8</a> —@SenSchumer

No law mandates that parents must be separated from their children at the border, and it's not a policy Democrats have pushed or can change alone as the minority in Congress. Chuck Schumer, minority leader in the Senate, quickly criticized Trump's latest tweet on the subject.

In recent years, U.S. administrations have been following a so-called "catch and release" policy. Persons eligible to apply for asylum have most often not been detained while their cases are heard, while those entering illegally have been processed for civil deportation, but most often not prosecuted, in part due to the backlog of cases and strain on the system.

The Trump administration announced in April its intention to end the catch-and-release policy, and since last year has been separating children from families and treating them similarly to the "unaccompanied minors" that can arrive in the U.S. It has been estimated that several hundred children have been separated as a result.

No 'moral right,' Sessions says

Attorney General Jeff Session publicly announced a zero tolerance policy for illegal entry in April, with cases being referred to for criminal prosecution.

"If you are smuggling a child then we will prosecute you, and that child will be separated from you as required by law," said Sessions. "If you don't like that, then don't smuggle children over our border."

A conviction for illegal entry carries a maximum penalty of six months in prison for first-time crossers and two years for repeat offences. In practice, many in recent years have been deported after pleading guilty, with some spending a few days in jail.

According to U.S. Customs and Border Protection, more than 650 children were separated from the parents at the border during a two-week period in May.

Attorney General Jeff Sessions wants more criminal prosecutions for those who enter illegally, even though it may lead to a tremendous backlog to hear cases, as well as the need for more temporary facilities to shelter children. (Susan Walsh/Associated Press)

Sessions told conservative radio host Hugh Hewitt on Tuesday that the U.S. doesn't have a "moral right" to keep the families together and stressed that most of the time the separations could be measured "within days."

"Now, if they get into a prolonged asylum process, the children are then turned over to some sort of family that is to take care of them while the adult may be in trial," Sessions said, according to a transcript of The Hugh Hewitt Show.

"But basically, the adults are frequently getting bail, too, and be able to be with their children."

According to an NBC News report on Tuesday, the U.S. Health and Human Services (HHS), an agency responsible for sheltering migrant children, was not given advance warning of the shift in administration policy. Military installations are being considered to shelter children as officials deal with a lack of existing space.

Sessions told Hewitt he had not visited any of the existing facilities where the children are being housed.

'Criminalization of asylum seekers'

The new strategy has been panned by human rights group as inhumane and by those working in the field as impractical.

"This is now the criminalization of asylum-seekers now being penalized for the way they're coming to the U.S. and seeking protection," Texas immigration lawyer Manoj Govindaiah told CBC News earlier this month.

Critics argue that separations are part of a poorly conceived and overly restrictive approach to immigration. The Trump administration has also signalled its intention to end Deferred Action for Childhood Arrivals (DACA) protections for those who are not American citizens but have been in the U.S. for years or even decades being brought to the country as children by their parents or other adults.

In this April 29 photo, Central Americans who travel with a caravan of migrants walk from Tijuana before crossing the border to the U.S. and requesting asylum in the United States (Hans-Maximo Musielik/Associated Press)

Refugee admissions in Trump's first year of office, meanwhile, were estimated at just under 30,000. It's the lowest total since 2002, when the U.S. was dealing with the fallout from the Sept. 11 attacks.

The administration has given shifting explanations for the need to separate children, including a desire to break up trafficking rings and the rising number of cases at the border.

Border arrests, a useful but imperfect gauge of illegal crossings, reached 50,308 in March, up 37 per cent from February and more than triple the same period than in 2017. That's still less than periodic surges during former President Barack Obama's second term and much lower than the 1990s and early 2000s.

The government is facing legal challenges due to the separations. The American Civil Liberties Union earlier this year sued on behalf of a Congolese woman who was separated from her seven-year-old daughter for five months after seeking asylum at a San Diego border crossing and a Brazilian asylum-seeker who has been separated from her 14-year-old son since an arrest for illegal entry in August near the Texas-New Mexico border.