US President Donald Trump has attacked the German government's approach to immigration, as he comes under pressure over his own crackdown on migrants.
German Chancellor Angela Merkel and US president Donald Trump earlier this month. Photo: AFP
Mr Trump said German Chancellor Angela Merkel's coalition looked "tenuous".
Mrs Merkel is in the middle of a serious domestic row after defending her government's immigration policy.
Meanwhile, Mr Trump has adopted a highly criticised "zero-tolerance" approach, which has split families crossing the Mexican border.
That position has been condemned by many, including former First Lady Laura Bush - wife of Republican ex-president George Bush - who wrote about it in the Washington Post.
"[Trump's] zero-tolerance policy is cruel. It is immoral. And it breaks my heart," she wrote, citing stories of children held in detention centres.
In a series of tweets, the president defended his stance. "We don't want what is happening with immigration in Europe to happen with us!" he said.
Meanwhile, Mrs Merkel is in a migration dispute with her interior minister, Horst Seehofer.
The chancellor has flatly refused to back Mr Seehofer's plan to turn away migrants at the German border if they have already registered elsewhere in the EU.
The disagreement has opened up a rift between Mrs Merkel's Christian Democrats and Mr Seehofer's Bavarian CSU party - two parties that have been closely aligned since the end of the World War II.
The backlash has left Mrs Merkel trying to hold her coalition together.
Other claims in Mr Trump's tweets have already been debunked by various German news sites, especially his assertion that "crime in Germany is way up".
They pointed out that Germany's crime rate is at its lowest since 1992, according to official figures released last month.
What is happening in the US?
President Trump said on Monday that the US "will not be a migrant camp and it will not be a refugee-holding facility". "Not on my watch."
Last month, Attorney-General Jeff Sessions introduced a policy to deter new arrivals.
The significant change, analysts say, is the justice department's decision to prosecute parents if they illegally cross the border, even if it is their first offence.
Adults who try to cross the border outside official entry points - many planning to seek asylum - are placed in custody and face criminal prosecution for illegal entry.
Their children - who are not charged with a crime - are taken to separate facilities, including warehouses and supermarkets that have been converted to deal with the swelling numbers.
On Monday, UN human rights chief Zeid Ra'ad al-Hussein called on the US to end what he called its "unconscionable" strategy of forced separations.
President Trump's wife, Melania, also made a rare statement expressing concern.
She said she "hates to see children separated from their families", according to her spokeswoman.
She said "both sides" - Republicans and Democrats - needed to come together to resolve the immigration problem.
Mr Trump has blamed the Democrats "for being weak and ineffective with Boarder [sic] Security and Crime".
Democrats - and numerous Republicans - have said the president's hardline approach is not the answer.
"Children shouldn't be used as a negotiating tool," tweeted Mr Trump's former presidential Republican rival Jeb Bush.
What is happening in Germany?
Mrs Merkel wants a new deal at the EU level over migrants. She was widely criticised domestically for letting in about one million in 2015.
Mr Seehofer has been a longstanding critic of that open-door policy, which was summed up at the time by Mrs Merkel's phrase "we can manage".
He said German border police should turn back all asylum seekers who lack IDs and those already registered in another EU country.
Mrs Merkel sees that as a "go-it-alone" policy which would further hurt Italy and Greece, countries whose capacity to house migrants is stretched to breaking point.
The chancellor has been holding emergency talks with her party.
There is a risk that Mr Seehofer's CSU party could break away, or at least trigger a confidence vote in Mrs Merkel's leadership.
- BBC