President Donald Trump, facing renewed criticism from Democrats and activists over his handling of a migrant crisis on the US- Mexico border, said in a Twitter post on Wednesday that immigrants unhappy with conditions at detention centres should be told “not to come.”

“If Illegal Immigrants are unhappy with the conditions in the quickly built or refitted detentions centers, just tell them not to come. All problems solved!” Trump said on Twitter.

The Republican president has made cracking down on illegal immigration a key part of his first-term agenda after campaigning on the issue ahead of the 2016 election.

Democratic lawmakers and civil rights activists who have visited migrant detention centres along the border in recent days have described nightmarish conditions marked by overcrowding and inadequate access to food, water and other basic needs.

The Department of Homeland Security’s inspector general on Tuesday published photos of migrant-holding centers in Texas’ Rio Grande Valley crammed with twice as many people as they were meant to hold.

“Our Border Patrol people are not hospital workers, doctors or nurses,” Trump wrote earlier on Twitter. “Great job by Border Patrol, above and beyond. Many of these illegals aliens are living far better now than where they came from, and in far safer conditions.”

Criticism of the US Customs and Border Protection agency grew after reports this week that current and former agents had posted offensive anti-immigrant comments and targeted lawmakers on their private Facebook group.

The Facebook posts, first reported by ProPublica, included jokes about immigrants dying and sexually explicit content about US Democratic Representative Alexandria Ocasio-Cortez, who criticized the detention facilities after a tour this week.Democratic Senate Minority Leader Chuck Schumer called for the acting head of the CBP and other top leaders at the agency to be fired.

Democratic US Representative Joaquin Castro said after a visit to the border this week that detainees had been not been allowed to bathe for two weeks, were deprived of medication and locked in areas with broken water faucets.