Winston Peters says the forced separation policy is a matter for the US.

Foreign Affairs Minister Winston Peters has expressed "concern" but not condemnation of United States President Donald Trump's policy of forcibly separating migrant children from their parents.

National and the Green Party have called the policy inhumane, with the Greens calling on the Government to condemn the policy.

Peters has been restrained on the matter, saying every cabinet minister was "concerned", but it was an internal matter for the US.

ABIGAIL DOUGHERTY/STUFF Deputy Prime Minister Winston Peters said it was a decision for the US, but we wouldn't do it here.

The Labour Party has made clear its discomfort, but a spokeswoman for Prime Minister Jacinda Ardern directed queries towards Peters' office, saying Ardern would not be commenting at this time.

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MONIQUE FORD/STUFF Simon Bridges speaks on US border policy.

Trump's zero tolerance immigration policy has resulted in nearly 2000 children being taken from their parents at the border, some being kept in chain-link cages. Haunting audio of children crying for their parents was briefly played by a reporter during a White House press briefing this week as backlash to the policy mounts.

National leader Simon Bridges said the policy looked "inhumane" on Wednesday morning.

"I put myself in the situation what would I do if I was a leader in that situation and we had an analogous situation in New Zealand I don't think I could stand for that," Bridges said.

JOHN MOORE/GETTY IMAGES A two-year-old Honduran asylum seeker cries as her mother is searched and detained.

"To see families separated, to see children separated from their parents - it's inhumane."

Peters was asked about the issue on Tuesday morning and said it was a decision "for the US congress and the US Supreme Court" - not him.

The new zero-tolerance policy is in fact led by the executive in the US, not Congress - although it relies on laws written by Congress.

"I could have views on every other country in the world, but we're trying to run this country," Peters said.

"Would we do that? No we wouldn't."

On Wednesday afternoon, Peters said there was "not a cabinet minister or member of Parliament who is not concerned about it" but he would not "have a contest to see who is most outraged."

"Every Cabinet minister and member of Parliament in this country is concerned with what is developing there," Peters said.

He has not written to the US ambassador about the issue but said "we always raise concerns like that when we meet personally and that will probably happen when the time comes."

Peters said it was an "internal debate" for the US that he would be watching with great interest.

GREENS URGE GOVERNMENT TO SPEAK OUT

The Green Party has struck out against the policy and called on the Government to condemn it.

"The mass detention of children and separation from their families as a part of immigration policy is unacceptable," Green human rights spokeswoman Golriz Ghahraman said.

"It is abhorrent to do it as a way to 'deter' their parents from seeking asylum or migrating to the United States, and New Zealand must condemn it.

"The Green Party is calling on the prime minister and minister for foreign affairs and trade to express New Zealand's opposition to this practice in the strongest possible terms to the United States ambassador."

Ghahraman said her office was in discussion with Peters' office on the matter.

She said there were other moments in history when people wish they had spoken out about policies like this - and this was one of those.

Labour's deputy leader Kelvin Davis said the policy was "cruel" on Tuesday.

"A family is a family and in my opinion families should be allowed to stick together. To me it's cruel. It's outright cruelty," Davis said.

He said the US should govern with "heart".

The prime Minister's office has been asked for comment.

United Kingdom Prime Minister Theresa May has made a similar comment as Peters, saying the UK would not enact a similar policy themselves but stopping short of outright criticism.

Canadian Prime Minister Justin Trudeau has also been muted on the matter.

A French Government spokesman said France and the US "clearly don't share certain values".