"On what we have seen in the United States, pictures of children being held in what appear to be cages are deeply disturbing... this is wrong," she told MPs.
May said she would raise the issue with Trump when the pair meet in Britain next month.
"When we disagree with the United States we tell them so," she told MPs.
"But we also have some key shared interests with the United States in the security and defence field and on other areas as well.
May appeared to criticise the practice, saying that when she was interior minister: "I ended the routine detention of families with children." Opposition Labour leader Jeremy Corbyn's spokesman said the US policy was "immoral, a fundamental abuse of human rights." "We're in favour of dialogue with international leaders... but the soft pedalling on these abuses of human rights -- and it's not restricted to this issue -- by the Trump administration has achieved nothing." The prime minister has been a frequent critic and the relationship between the two appears to have soured after May became the first world leader to visit Trump's White House last February.
She spoke out against his travel ban on Muslim-majority countries, his decisions to leave the Paris climate accord and the Iran nuclear deal as well as his imposition of trade tariffs.
Relations between the pair were further strained in November after Trump re-tweeted anti-Muslim videos posted by a British far-right group.
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