US first lady Melania Trump has spoken out against the detention of thousands of migrant children separated from their parents by American authorities at the US-Mexico border.

US officials said on Friday that nearly 2000 children were separated from adults at the border between mid-April and the end of May.

In a rare comment, Mrs Trump spoke against the Trump administration’s harsh policy, saying the US needs to be “a country that governs with heart”.

“Mrs Trump hates to see children separated from their families and hopes both sides of the aisle can finally come together to achieve successful immigration reform,” Mrs Trump’s communications director Stephanie Grisham said in a statement to CNN.

“She believes we need to be a country that follows all laws, but also a country that governs with heart.”

Mrs Trump’s statement comes as Democratic lawmakers joined hundreds of protesters outside an immigration detention facility in New Jersey for a Father’s Day demonstration.

Demonstrators gather outside the Massachusetts State House in Boston to protest against the Trump administration policy. Photo: Getty

“This must not be who we are as a nation,” Representative Jerrold Nadler said, one of seven members of congress from New York and New Jersey who met with five detainees inside the facility, including three who said they had young relatives removed from their care after seeking asylum at the border.

The event in the city of Elizabeth on Sunday came as news stories highlighting the family separations intensified political pressure on the White House, even from some of President Donald Trump’s fellow Republicans.

In May, US Attorney General Jeff Sessions announced a “zero tolerance” policy in which all those apprehended entering the United States illegally, including those seeking asylum, would be criminally charged, which generally leads to children being separated from their parents.

The policy has drawn condemnation from medical professionals, religious leaders and immigration activists, who warn that some children could suffer lasting psychological trauma.

The children are held in government facilities, released to adult sponsors or placed in temporary foster care.

In South Texas on Sunday, several Democratic lawmakers, including Senator Jeff Merkley, were due to tour detention facilities to call attention to the policy, while Representative Beto O’Rourke, who is running for the US Senate in Texas, said he would march with protesters to the border.

A two-year-old Honduran asylum seeker cries as her mother is searched and detained near the US-Mexico border on June 12 in McAllen, Texas. Photo: Getty

“This is inhumane,” Mr O’Rourke told CNN. “I’d like to say it’s un-American, but it’s happening right now in America.”

Some moderate Republicans have also called on Mr Trump to stop the separations. Senators Susan Collins and Jeff Flake wrote to White House officials on Saturday seeking more information on the policy.

“It is inconsistent with our American values to separate these children from their parents,” Ms Collins said on CBS’ “Face the Nation” on Sunday.

Administration officials have defended the tactic as necessary to secure the border and suggested it would act as a deterrent to illegal immigration, while Mr Trump has sought to blame Democrats, saying their support for passage of a broader immigration bill would end the separations.

Democrats can fix their forced family breakup at the Border by working with Republicans on new legislation, for a change! This is why we need more Republicans elected in November. Democrats are good at only three things, High Taxes, High Crime and Obstruction. Sad!

— Donald J. Trump (@realDonaldTrump) June 16, 2018

“As a mother, as a Catholic, as somebody who has got a conscience … I will tell you that nobody likes this policy,” White House adviser Kellyanne Conway said on NBC’s Meet the Press on Sunday.

“You saw the President [say] on camera that he wants this to end.”

Democrats have accused Mr Trump of effectively turning the children into political hostages to secure stricter immigration measures, such as funding for a US-Mexico border wall.

-with AAP